# Package org.apache.commons.beanutils

package org.apache.commons.beanutils

The Bean Introspection Utilities component of the Apache Commons subproject offers low-level utility classes that assist in getting and setting property values on Java classes that follow the naming design patterns outlined in the JavaBeans Specification, as well as mechanisms for dynamically defining and accessing bean properties.

## Table of Contents

- 1. Overview
  - 1.1 Background
  - 1.2 External Dependencies
- 2. Standard JavaBeans
  - 2.1 Background
  - 2.2 Basic Property Access
  - 2.3 Nested Property Access
  - 2.4 Customizing Introspection
  - 2.5 Suppressing Properties
- 3. Dynamic Beans (DynaBeans)
  - 3.1 Background
  - 3.2 BasicDynaBean and BasicDynaClass
  - 3.3 ResultSetDynaClass (Wraps ResultSet in DynaBeans)
  - 3.4 RowSetDynaClass (Disconnected ResultSet as DynaBeans)
  - 3.5 WrapDynaBean and WrapDynaClass
  - 3.6 Lazy DynaBeans
- 4. Data Type Conversions
  - 4.1 Background
  - 4.2 BeanUtils and ConvertUtils Conversions
  - 4.3 Defining Your Own Converters
  - 4.4 Locale Aware Conversions
- 5. Utility Objects And Static Utility Classes
- 6. Collections
  - 6.1 Comparing Beans
  - 6.2 Operating On Collections Of Beans
  - 6.3 Querying Or Filtering Collections Of Beans
  - 6.4 Transforming Collections Of Beans
- 7. Frequently Asked Questions
  - Why Can't BeanUtils Find My Method?
  - How Do I Set The BeanComparator Order To Be Ascending/Descending?

## 1. Overview

## 1.1 Background

The JavaBeans name comes from a Java API for a component architecture for the Java language. Writing Java classes that conform to the JavaBeans design patterns makes it easier for Java developers to understand the functionality provided by your class, as well as allowing JavaBeans-aware tools to use Java's introspection capabilities to learn about the properties and operations provided by your class, and present them in a visually appealing manner in development tools.

The JavaBeans Specification describes the complete set of characteristics that makes an arbitrary Java class a JavaBean or not -- and you should consider reading this document to be an important part of developing your Java programming skills. However, the required characteristics of JavaBeans that are important for most development scenarios are listed here:

- The class must be public, and provide a public constructor that accepts no arguments. This allows tools and applications to dynamically create new instances of your bean, without necessarily knowing what Java class name will be used ahead of time, like this:

               String className = ...;
               Class beanClass = Class.forName(className);
               Object beanInstance = beanClass.newInstance();

- As a necessary consequence of having a no-arguments constructor, configuration of your bean's behavior must be accomplished separately from its instantiation. This is typically done by defining a set of properties of your bean, which can be used to modify its behavior or the data that the bean represents. The normal convention for property names is that they start with a lower case letter, and be comprised only of characters that are legal in a Java identifier.
- Typically, each bean property will have a public getter and setter method that are used to retrieve or define the property's value, respectively. The JavaBeans Specification defines a design pattern for these names, using `get` or `set` as the prefix for the property name with it's first character capitalized. Thus, you a JavaBean representing an employee might have (among others) properties named `firstName`, `lastName`, and `hireDate`, with method signatures like this:

               public class Employee {
                   public Employee();   // Zero-arguments constructor
                   public String getFirstName();
                   public void setFirstName(String firstName);
                   public String getLastName();
                   public void setLastName(String lastName);
                   public Date getHireDate();
                   public void setHireDate(Date hireDate);
                   public boolean isManager();
                   public void setManager(boolean manager);
                   public String getFullName();
               }

- As you can see from the above example, there is a special variant allowed for boolean properties -- you can name the getter method with a `is` prefix instead of a `get` prefix if that makes for a more understandable method name.
- If you have both a getter and a setter method for a property, the data type returned by the getter must match the data type accepted by the setter. In addition, it is contrary to the JavaBeans specification to have more than one setter with the same name, but different property types.
- It is not required that you provide a getter and a setter for every property. In the example above, the `fullName` property is read-only, because there is no setter method. It is also possible, but less common, to provide write-only properties.
- It is also possible to create a JavaBean where the getter and setter methods do not match the naming pattern described above. The standard JavaBeans support classes in the Java language, as well as all classes in the BeanUtils package, allow you to describe the actual property method names in a `BeanInfo` class associated with your bean class. See the JavaBeans Specification for full details.
- The JavaBeans Specification also describes many additional design patterns for event listeners, wiring JavaBeans together into component hierarchies, and other useful features that are beyond the scope of the BeanUtils package.

Using standard Java coding techniques, it is very easy to deal with JavaBeans if you know ahead of time which bean classes you will be using, and which properties you are interested in:

             Employee employee = ...;
             System.out.println("Hello " + employee.getFirstName() + "!");

## 1.2 External Dependencies

The commons-beanutils package requires that the following additional packages be available in the application's class path at runtime:

- Logging Package (Apache Commons), version 1.0 or later
- Collections Package (Apache Commons), version 1.0 or later

## 2. Standard JavaBeans

## 2.1 Background

As described above, the standard facilities of the Java programming language make it easy and natural to access the property values of your beans using calls to the appropriate getter methods. But what happens in more sophisticated environments where you do not necessarily know ahead of time which bean class you are going to be using, or which property you want to retrieve or modify? The Java language provides classes like `java.beans.Introspector`, which can examine a Java class at runtime and identify for you the names of the property getter and setter methods, plus the Reflection capabilities to dynamically call such a method. However, these APIs can be difficult to use, and expose the application developer to many unnecessary details of the underlying structure of Java classes. The APIs in the BeanUtils package are intended to simplify getting and setting bean properties dynamically, where the objects you are accessing -- and the names of the properties you care about -- are determined at runtime in your application, rather than as you are writing and compiling your application's classes.

This is the set of needs that are satisfied by the static methods of the `PropertyUtils` class, which are described further in this section. First, however, some further definitions will prove to be useful:

The general set of possible property types supported by a JavaBean can be broken into three categories -- some of which are supported by the standard JavaBeans specification, and some of which are uniquely supported by the BeanUtils package:

- Simple - Simple, or scalar, properties have a single value that may be retrieved or modified. The underlying property type might be a Java language primitive (such as `int`, a simple object (such as a `java.lang.String`), or a more complex object whose class is defined either by the Java language, by the application, or by a class library included with the application.
- Indexed - An indexed property stores an ordered collection of objects (all of the same type) that can be individually accessed by an integer-valued, non-negative index (or subscript). Alternatively, the entire set of values may be set or retrieved using an array. As an extension to the JavaBeans specification, the BeanUtils package considers any property whose underlying data type is `java.util.List` (or an implementation of List) to be indexed as well.
- Mapped - As an extension to standard JavaBeans APIs, the BeanUtils package considers any property whose underlying value is a `java.util.Map` to be "mapped". You can set and retrieve individual values via a String-valued key.

A variety of API methods are provided in the `PropertyUtils` class to get and set property values of all of these types. In the code fragments below, assume that there are two bean classes defined with the following method signatures:

         public class Employee {
             public Address getAddress(String type);
             public void setAddress(String type, Address address);
             public Employee getSubordinate(int index);
             public void setSubordinate(int index, Employee subordinate);
             public String getFirstName();
             public void setFirstName(String firstName);
             public String getLastName();
             public void setLastName(String lastName);
         }

## 2.2 Basic Property Access

Getting and setting simple property values is, well, simple :-). Check out the following API signatures in the Javadocs:

- `PropertyUtils.getSimpleProperty(Object, String)`
- `PropertyUtils.setSimpleProperty(Object, String, Object)`

Using these methods, you might dynamically manipulate the employee's name in an application:

         Employee employee = ...;
         String firstName = (String)
           PropertyUtils.getSimpleProperty(employee, "firstName");
         String lastName = (String)
           PropertyUtils.getSimpleProperty(employee, "lastName");
         ... manipulate the values ...
         PropertyUtils.setSimpleProperty(employee, "firstName", firstName);
         PropertyUtils.setSimpleProperty(employee, "lastName", lastName);

For indexed properties, you have two choices - you can either build a subscript into the "property name" string, using square brackets, or you can specify the subscript in a separate argument to the method call:

- `PropertyUtils.getIndexedProperty(Object, String)`
- `PropertyUtils.getIndexedProperty(Object, String, int)`
- `PropertyUtils.setIndexedProperty(Object, String, Object)`
- `PropertyUtils.setIndexedProperty(Object, String, int, Object)`

Only integer constants are allowed when you add a subscript to the property name. If you need to calculate the index of the entry you wish to retrieve, you can use String concatenation to assemble the property name expression. For example, you might do either of the following:

         Employee employee = ...;
         int index = ...;
         String name = "subordinate[" + index + "]";
         Employee subordinate = (Employee)
           PropertyUtils.getIndexedProperty(employee, name);

         Employee employee = ...;
         int index = ...;
         Employee subordinate = (Employee)
           PropertyUtils.getIndexedProperty(employee, "subordinate", index);

In a similar manner, there are two possible method signatures for getting and setting mapped properties. The difference is that the extra argument is surrounded by parentheses ("(" and ")") instead of square brackets, and it is considered to be a String-value key used to get or set the appropriate value from an underlying map.

- `PropertyUtils.getMappedProperty(Object, String)`
- `PropertyUtils.getMappedProperty(Object, String, String)`
- `PropertyUtils.setMappedProperty(Object, String, Object)`
- `PropertyUtils.setMappedProperty(Object, String, String, Object)`

You can, for example, set the employee's home address in either of these two manners:

         Employee employee = ...;
         Address address = ...;
         PropertyUtils.setMappedProperty(employee, "address(home)", address);

         Employee employee = ...;
         Address address = ...;
         PropertyUtils.setMappedProperty(employee, "address", "home", address);

## 2.3 Nested Property Access

In all of the examples above, we have assumed that you wished to retrieve the value of a property of the bean being passed as the first argument to a PropertyUtils method. However, what if the property value you retrieve is really a Java object, and you wish to retrieve a property of that object instead?

For example, assume we really wanted the `city` property of the employee's home address. Using standard Java programming techniques for direct access to the bean properties, we might write:

         String city = employee.getAddress("home").getCity();

The equivalent mechanism using the PropertyUtils class is called nested property access. To use this approach, you concatenate together the property names of the access path, using "." separators -- very similar to the way you can perform nested property access in JavaScript.

- `PropertyUtils.getNestedProperty(Object, String)`
- `PropertyUtils.setNestedProperty(Object, String, Object)`

The PropertyUtils equivalent to the above Java expression would be:

         String city = (String)
           PropertyUtils.getNestedProperty(employee, "address(home).city");

Finally, for convenience, PropertyUtils provides method signatures that accept any arbitrary combination of simple, indexed, and mapped property access, using any arbitrary level of nesting:

- `PropertyUtils.getProperty(Object, String)`
- `PropertyUtils.setProperty(Object, String, Object)`

which you might use like this:

         Employee employee = ...;
         String city = (String) PropertyUtils.getProperty(employee,
           "subordinate[3].address(home).city");

## 2.4 Customizing Introspection

As was pointed out, BeanUtils relies on conventions defined by the JavaBeans specification to determine the properties available for a specific bean class. Thus all classes conforming to these conventions can be used out of the box.

Sometimes an application has to deal with classes using different conventions. For instance, fluent APIs allowing method chaining are not compliant to the JavaBeans specification because here set methods have non-void return values. From version 1.9.0 onwards, BeanUtils supports customization of its introspection mechanism. This allows an application to extend or modify the default discovery of bean properties.

The key to this extension mechanism is the `BeanIntrospector` interface. The purpose of an object implementing this interface is to process a specific target class and create corresponding `PropertyDescriptor` objects for the properties it detects. Per default, BeanUtils uses a `DefaultBeanIntrospector` object which detects properties compatible with the JavaBeans specification.

In order to extend the property discovery mechanism, `PropertyUtils` offers the `PropertyUtils.addBeanIntrospector(BeanIntrospector)` method. Here a custom `BeanIntrospector` implementation can be passed in. During introspection of a class, this custom introspector is then called, and it can add the properties it has detected to the total result. As an example of such a custom `BeanIntrospector` implementation, BeanUtils ships with the `FluentPropertyBeanIntrospector` class. This implementation can detect properties whose set methods have a non-void return type - thus enabling support for typical properties in a fluent API.

## 2.5 Suppressing Properties

The mechanism of customizing bean introspection described in the previous section can also be used to suppress specific properties. There is a specialized `BeanIntrospector` implementation that does exactly this: `SuppressPropertiesBeanIntrospector`. When creating an instance, a collection with the names of properties that should not be accessible on beans has to be provided. These properties will then be removed if they have been detected by other `BeanIntrospector` instances during processing of a bean class.

A good use case for suppressing properties is the special `class` property which is per default available for all beans; it is generated from the `getClass()` method inherited from `Object` which follows the naming conventions for property get methods. Exposing this property in an uncontrolled way can lead to a security vulnerability as it allows access to the class loader. More information can be found at https://issues.apache.org/jira/browse/BEANUTILS-463.

Because the `class` property is undesired in many use cases there is already an instance of `SuppressPropertiesBeanIntrospector` which is configured to suppress this property. It can be obtained via the `SUPPRESS_CLASS` constant of `SuppressPropertiesBeanIntrospector`.

## 3. Dynamic Beans (DynaBeans)

## 3.1 Background

The `PropertyUtils` class described in the preceding section is designed to provide dynamic property access on existing JavaBean classes, without modifying them in any way. A different use case for dynamic property access is when you wish to represent a dynamically calculated set of property values as a JavaBean, but without having to actually write a Java class to represent these properties. Besides the effort savings in not having to create and maintain a separate Java class, this ability also means you can deal with situations where the set of properties you care about is determined dynamically (think of representing the result set of an SQL select as a set of JavaBeans ...).

To support this use case, the BeanUtils package provides the `DynaBean` interface, which must be implemented by a bean class actually implementing the interface's methods, and the associated `DynaClass` interface that defines the set of properties supported by a particular group of DynaBeans, in much the same way that `java.lang.Class` defines the set of properties supported by all instances of a particular JavaBean class.

For example, the `Employee` class used in the examples above might be implemented as a DynaBean, rather than as a standard JavaBean. You can access its properties like this:

         DynaBean employee = ...; // Details depend on which
                                  // DynaBean implementation you use
         String firstName = (String) employee.get("firstName");
         Address homeAddress = (Address) employee.get("address", "home");
         Object subordinate = employee.get("subordinate", 2);

One very important convenience feature should be noted: the PropertyUtils property getter and setter methods understand how to access properties in DynaBeans. Therefore, if the bean you pass as the first argument to, say, `PropertyUtils.getSimpleProperty()` is really a DynaBean implementation, the call will get converted to the appropriate DynaBean getter method transparently. Thus, you can base your application's dynamic property access totally on the PropertyUtils APIs, if you wish, and use them to access either standard JavaBeans or DynaBeans without having to care ahead of time how a particular bean is implemented.

Because DynaBean and DynaClass are interfaces, they may be implemented multiple times, in different ways, to address different usage scenarios. The following subsections describe the implementations that are provided as a part of the standard BeanUtils package, although you are encouraged to provide your own custom implementations for cases where the standard implementations are not sufficient.

## 3.2 `BasicDynaBean` and `BasicDynaClass`

The `BasicDynaBean` and `BasicDynaClass` implementation provides a basic set of dynamic property capabilities where you want to dynamically define the set of properties (described by instances of `DynaProperty`). You start by defining the DynaClass that establishes the set of properties you care about:

         DynaProperty[] props = new DynaProperty[]{
             new DynaProperty("address", java.util.Map.class),
             new DynaProperty("subordinate", mypackage.Employee[].class),
             new DynaProperty("firstName", String.class),
             new DynaProperty("lastName",  String.class)
           };
         BasicDynaClass dynaClass = new BasicDynaClass("employee", null, props);

Note that the 'dynaBeanClass' argument (in the constructor of `BasicDynaClass`) can have the value of `null`. In this case, the value of `dynaClass.getDynaBeanClass` will just be the `Class` for BasicDynaBean.

Next, you use the `newInstance()` method of this DynaClass to create new DynaBean instances that conform to this DynaClass, and populate its initial property values (much as you would instantiate a new standard JavaBean and then call its property setters):

         DynaBean employee = dynaClass.newInstance();
         employee.set("address", new HashMap());
         employee.set("subordinate", new mypackage.Employee[0]);
         employee.set("firstName", "Fred");
         employee.set("lastName", "Flintstone");

Note that the DynaBean class was declared to be `DynaBean` instead of `BasicDynaBean`. In general, if you are using DynaBeans, you will not want to care about the actual implementation class that is being used -- you only care about declaring that it is a `DynaBean` so that you can use the DynaBean APIs.

As stated above, you can pass a DynaBean instance as the first argument to a `PropertyUtils` method that gets and sets properties, and it will be interpreted as you expect -- the dynamic properties of the DynaBean will be retrieved or modified, instead of underlying properties on the actual BasicDynaBean implementation class.

## 3.3 `ResultSetDynaClass` (Wraps ResultSet in DynaBeans)

A very common use case for DynaBean APIs is to wrap other collections of "stuff" that do not normally present themselves as JavaBeans. One of the most common collections that would be nice to wrap is the `java.sql.ResultSet` that is returned when you ask a JDBC driver to perform a SQL SELECT statement. Commons BeanUtils offers a standard mechanism for making each row of the result set visible as a DynaBean, which you can utilize as shown in this example:

       Connection conn = ...;
       Statement stmt = conn.createStatement();
       ResultSet rs = stmt.executeQuery
         ("select account_id, name from customers");
       Iterator rows = (new ResultSetDynaClass(rs)).iterator();
       while (rows.hasNext()) {
         DynaBean row = (DynaBean) rows.next();
         System.out.println("Account number is " +
                            row.get("account_id") +
                            " and name is " + row.get("name"));
       }
       rs.close();
       stmt.close();

## 3.4 `RowSetDynaClass` (Disconnected ResultSet as DynaBeans)

Although `ResultSetDynaClass` is a very useful technique for representing the results of an SQL query as a series of DynaBeans, an important problem is that the underlying `ResultSet` must remain open throughout the period of time that the rows are being processed by your application. This hinders the ability to use `ResultSetDynaClass` as a means of communicating information from the model layer to the view layer in a model-view-controller architecture such as that provided by the Struts Framework, because there is no easy mechanism to assure that the result set is finally closed (and the underlying `Connection` returned to its connection pool, if you are using one).

The `RowSetDynaClass` class represents a different approach to this problem. When you construct such an instance, the underlying data is copied into a set of in-memory DynaBeans that represent the result. The advantage of this technique, of course, is that you can immediately close the ResultSet (and the corresponding Statement), normally before you even process the actual data that was returned. The disadvantage, of course, is that you must pay the performance and memory costs of copying the result data, and the result data must fit entirely into available heap memory. For many environments (particularly in web applications), this tradeoff is usually quite beneficial.

As an additional benefit, the `RowSetDynaClass` class is defined to implement `java.io.Serializable`, so that it (and the DynaBeans that correspond to each row of the result) can be conveniently serialized and deserialized (as long as the underlying column values are also Serializable). Thus, `RowSetDynaClass` represents a very convenient way to transmit the results of an SQL query to a remote Java-based client application (such as an applet).

The normal usage pattern for a `RowSetDynaClass` will look something like this:

         Connection conn = ...;  // Acquire connection from pool
         Statement stmt = conn.createStatement();
         ResultSet rs = stmt.executeQuery("SELECT ...");
         RowSetDynaClass rsdc = new RowSetDynaClass(rs);
         rs.close();
         stmt.close();
         ...;                    // Return connection to pool
         List rows = rsdc.getRows();
         ...;                   // Process the rows as desired

## 3.5 `WrapDynaBean` and `WrapDynaClass`

OK, you've tried the DynaBeans APIs and they are cool -- very simple `get()` and `set()` methods provide easy access to all of the dynamically defined simple, indexed, and mapped properties of your DynaBeans. You'd like to use the DynaBean APIs to access all of your beans, but you've got a bunch of existing standard JavaBeans classes to deal with as well. This is where the `WrapDynaBean` (and its associated `WrapDynaClass`) come into play. As the name implies, a WrapDynaBean is used to "wrap" the DynaBean APIs around an existing standard JavaBean class. To use it, simply create the wrapper like this:

         MyBean bean = ...;
         DynaBean wrapper = new WrapDynaBean(bean);
         String firstName = wrapper.get("firstName");

Note that, although appropriate `WrapDynaClass` instances are created internally, you never need to deal with them.

## 3.6 Lazy DynaBeans

- 1. LazyDynaBean - A Lazy `DynaBean`
- 2. LazyDynaMap - A light weight `DynaBean` facade to a Map with lazy map/list processing
- 3. LazyDynaList - A lazy list for `DynaBean's`, `java.util.Map`'s or POJO beans.
- 4. LazyDynaClass - A `MutableDynaClass` implementation.

You bought into the DynaBeans because it saves coding all those POJO JavaBeans but you're here because lazy caught your eye and wondered whats that about? What makes these flavors of DynaBean lazy are the following features:

- Lazy property addition - lazy beans use a `DynaClass` which implements the `MutableDynaClass` interface. This provides the ability to add and remove a DynaClass's properties. Lazy beans use this feature to automatically add a property which doesn't exist to the DynaClass when the `set(name, value)` method is called.
- Lazy List/Array growth - If an indexed property is not large enough to accommodate the `index` being set then the `List` or `Array` is automatically grown so that it is.
- Lazy List/Array instantiation - if an indexed property doesn't exist then calling the `DynaBean's` indexed property getter/setter methods (i.e. `get(name, index)` or `set(name, index, value)`) results in either a new `List` or `Array` being instantiated. If the indexed property has not been defined in the DynaClass then it is automatically added and a default `List` implementation instantiated.
- Lazy Map instantiation - if a mapped property doesn't exist then calling the `DynaBean's` mapped property getter/setter methods (i.e. `get(name, key)` or `set(name, key, value)`) results in a new `Map` being instantiated. If the mapped property has not been defined in the DynaClass then it is automatically added and a default `Map` implementation instantiated.
- Lazy Bean instantiation - if a property is defined in the `DynaClass` as a `DynaBean` or regular bean and doesn't exist in the `DynaBean` then `LazyDynaBean` wiill try to instantiate the bean using a default empty constructor.

1. `LazyDynaBean` is the standard lazy bean implementation. By default it is associated with a `LazyDynaClass` which implements the `MutableDynaClass` interface - however it can be used with any `MutableDynaClass` implementation. The question is how do I use it? - well it can be as simple as creating a new bean and then calling the getters/setters...

         DynaBean dynaBean = new LazyDynaBean();

         dynaBean.set("foo", "bar");                   // simple

         dynaBean.set("customer", "title", "Mr");      // mapped
         dynaBean.set("customer", "surname", "Smith"); // mapped

         dynaBean.set("address", 0, addressLine1);     // indexed
         dynaBean.set("address", 1, addressLine2);     // indexed
         dynaBean.set("address", 2, addressLine3);     // indexed

2. `LazyDynaMap` is a light weight `DynaBean` facade to a `Map` with all the usual lazy features. Its light weight because it doesn't have an associated `DynaClass` containing all the properties. In fact it actually implements the `DynaClass` interface itself (and `MutableDynaClass`) and derives all the DynaClass information from the actual contents of the `Map`. A `LazyDynaMap` can be created around an existing `Map` or can instantiate its own `Map`. After any `DynaBean` processing has finished the `Map` can be retrieved and the DynaBean facade discarded.

If you need a new `Map` then to use....

         DynaBean dynaBean = new LazyDynaMap();        // create DynaBean

         dynaBean.set("foo", "bar");                   // simple
         dynaBean.set("customer", "title", "Mr");      // mapped
         dynaBean.set("address", 0, addressLine1);     // indexed

         Map myMap = dynaBean.getMap()                 // retrieve the Map

or to use with an existing `Map` ....

         Map myMap = ....                             // exisitng Map
         DynaBean dynaBean = new LazyDynaMap(myMap);  // wrap Map in DynaBean
         dynaBean.set("foo", "bar");                  // set properties

3. `LazyDynaList` is lazy list for `DynaBeans` `java.util.Map`'s or POJO beans. See the Javadoc for more details and example usage.

4. `LazyDynaClass` extends `BasicDynaClass` and implements the MutableDynaClass interface. It can be used with other `DynaBean` implementations, but it is the default `DynaClass` used by `LazyDynaBean`. When using the `LazyDynaBean` there may be no need to have anything to do with the `DynaClass`. However sometimes there is a requirement to set up the `DynaClass` first - perhaps to define the type of array for an indexed property, or if using the DynaBean in restricted mode (see note below) is required. Doing so is straight forward...

Either create a `LazyDynaClass` first...

         MutableDynaClass dynaClass = new LazyDynaClass();    // create DynaClass

         dynaClass.add("amount", java.lang.Integer.class);    // add property
         dynaClass.add("orders", OrderBean[].class);          // add indexed property
         dynaClass.add("orders", java.util.TreeMapp.class);   // add mapped property

         DynaBean dynaBean = new LazyDynaBean(dynaClass);     // Create DynaBean with associated DynaClass

or create a `LazyDynaBean` and get the `DynaClass`...

         DynaBean dynaBean = new LazyDynaBean();              // Create LazyDynaBean
         MutableDynaClass dynaClass =
                  (MutableDynaClass)dynaBean.getDynaClass();  // get DynaClass

         dynaClass.add("amount", java.lang.Integer.class);    // add property
         dynaClass.add("myBeans", myPackage.MyBean[].class);  // add 'array' indexed property
         dynaClass.add("myMap", java.util.TreeMapp.class);    // add mapped property

NOTE: One feature of `MutableDynaClass` is that it has a Restricted property. When the DynaClass is restricted no properties can be added or removed from the `DynaClass`. Neither the `LazyDynaBean` or `LazyDynaMap` will add properties automatically if the `DynaClass` is restricted.

## 4. Data Type Conversions

### 4.1 Background

So far, we've only considered the cases where the data types of the dynamically accessed properties are known, and where we can use Java casts to perform type conversions. What happens if you want to automatically perform type conversions when casting is not possible? The BeanUtils package provides a variety of APIs and design patterns for performing this task as well.

### 4.2 `BeanUtils` and `ConvertUtils` Conversions

A very common use case (and the situation that caused the initial creation of the BeanUtils package) was the desire to convert the set of request parameters that were included in a `javax.servlet.HttpServletRequest` received by a web application into a set of corresponding property setter calls on an arbitrary JavaBean. (This is one of the fundamental services provided by the Struts Framework, which uses BeanUtils internally to implement this functionality.)

In an HTTP request, the set of included parameters is made available as a series of String (or String array, if there is more than one value for the same parameter name) instances, which need to be converted to the underlying data type. The `BeanUtils` class provides property setter methods that accept String values, and automatically convert them to appropriate property types for Java primitives (such as `int` or `boolean`), and property getter methods that perform the reverse conversion. Finally, a `populate()` method is provided that accepts a `java.util.Map` containing a set of property values (keyed by property name), and calls all of the appropriate setters whenever the underlying bean has a property with the same name as one of the request parameters. So, you can perform the all-in-one property setting operation like this:

         HttpServletRequest request = ...;
         MyBean bean = ...;
         HashMap map = new HashMap();
         Enumeration names = request.getParameterNames();
         while (names.hasMoreElements()) {
           String name = (String) names.nextElement();
           map.put(name, request.getParameterValues(name));
         }
         BeanUtils.populate(bean, map);

The `BeanUtils` class relies on conversion methods defined in the `ConvertUtils` class to perform the actual conversions, and these methods are available for direct use as well. WARNING - It is likely that the hard coded use of `ConvertUtils` methods will be deprecated in the future, and replaced with a mechanism that allows you to plug in your own implementations of the `Converter` interface instead. Therefore, new code should not be written with reliance on ConvertUtils.

### 4.3 Defining Your Own Converters

The `ConvertUtils` class supports the ability to define and register your own String to Object conversions for any given Java class. Once registered, such converters will be used transparently by all of the `BeanUtils` methods (including `populate()`). To create and register your own converter, follow these steps:

- Write a class that implements the `Converter` interface. The `convert()` method should accept the `java.lang.Class` object of your application class (i.e. the class that you want to convert to, and a String representing the incoming value to be converted.
- At application startup time, register an instance of your converter class by calling the `ConvertUtils.register()` method.

### 4.4 Locale Aware Conversions

The standard classes in `org.apache.commons.beanutils` are not locale aware. This gives them a cleaner interface and makes then easier to use in situations where the locale is not important.

Extended, locale-aware analogues can be found in `org.apache.commons.beanutils.locale` . These are built along the same lines as the basic classes but support localization.

## 5. Utility Objects And Static Utility Classes

### Background

So far, the examples have covered the static utility classes (`BeanUtils`, `ConvertUtils` and `PropertyUtils`). These are easy to use but are somewhat inflexible. These all share the same registered converters and the same caches.

This functionality can also be accessed through utility objects (in fact, the static utility class use worker instances of these classes). For each static utility class, there is a corresponding class with the same functionality that can be instantiated:

Utility classes
Static Utility Class | Utility Object
--- | ---
BeanUtils | BeanUtilsBean
ConvertUtils | ConvertUtilsBean
PropertyUtils | PropertyUtilsBean

Creating an instances allow gives guarenteed control of the caching and registration to the code that creates it.

## 6. Collections

### 6.1 Comparing Beans

`org.apache.commons.beanutils.BeanComparator` is a `Comparator` implementation that compares beans based on a shared property value.

### 6.2 Operating On Collections Of Beans

The `Closure` interface in `commons-collections` encapsulates a block of code that executes on an arbitrary input Object. `Commons-collections` contains code that allows `Closures` to be applied to the contents of a Collection. For more details, see the commons-collections documentation.

`BeanPropertyValueChangeClosure` is a `Closure` that sets a specified property to a particular value. A typical usage is to combine this with `commons-collections` so that all the beans in a collection can have a particular property set to a particular value.

For example, set the activeEmployee property to TRUE for an entire collection:

         // create the closure
         BeanPropertyValueChangeClosure closure =
             new BeanPropertyValueChangeClosure( "activeEmployee", Boolean.TRUE );

         // update the Collection
         CollectionUtils.forAllDo( peopleCollection, closure );

### 6.3 Querying Or Filtering Collections Of Beans

The `Predicate` interface in `commons-collections` encapsulates an evaluation of an input Object that returns either true or false. `Commons-collections` contains code that allows `Predicates` to be applied to be used to filter collections. For more details, see the commons-collections documentation.

`BeanPropertyValueEqualsPredicate` is a `Predicate` that evaluates a set property value against a given value. A typical usage is (in combination with `commons-collections`) to filter collections on the basis of a property value.

For example, to filter a collection to find all beans where active employee is false use:

         BeanPropertyValueEqualsPredicate predicate =
             new BeanPropertyValueEqualsPredicate( "activeEmployee", Boolean.FALSE );

         // filter the Collection
         CollectionUtils.filter( peopleCollection, predicate );

### 6.4 Transforming Collections Of Beans

The `Transformer` interface in `commons-collections` encapsulates the transformation of an input Object into an output object. `Commons-collections` contains code that allows `Transformers` to be applied produce a collection of outputs from a collection of inputs. For more details, see the commons-collections documentation.

`BeanToPropertyTransformer` is a `Transformer` implementation that transforms a bean into it's property value.

For example, to find all cities that are contained in the address of each person property of each bean in a collection:

         // create the transformer
         BeanToPropertyValueTransformer transformer = new BeanToPropertyValueTransformer( "person.address.city" );

         // transform the Collection
         Collection peoplesCities = CollectionUtils.collect( peopleCollection, transformer );

## 7. Frequently Asked Questions

### Why Can't BeanUtils Find My Method?

The BeanUtils package relies on introspection rather than reflection. This means that it will find only JavaBean compliant properties.

There are some subtleties of this specification that can catch out the unwary:

- A property can have only one set and one get method. Overloading is not allowed.
- The `java.beans.Introspector` searches widely for a custom BeanInfo class. If your class has the same name as another with a custom BeanInfo (typically a java API class) then the `Introspector` may use that instead of creating via reflection based on your class. If this happens, the only solution is to create your own BeanInfo.

### How Do I Set The BeanComparator Order To Be Ascending/Descending?

BeanComparator relies on an internal Comparator to perform the actual comparisions. By default, `org.apache.commons.collections.comparators.ComparableComparator` is used which imposes a natural order. If you want to change the order, then a custom Comparator should be created and passed into the appropriate constructor.

For example:

         import org.apache.commons.collections.comparators.ComparableComparator;
         import org.apache.commons.collections.comparators.ReverseComparator;
         import org.apache.commons.beanutils.BeanComparator;
         ...
         BeanComparator reversedNaturalOrderBeanComparator
             = new BeanComparator("propertyName", new ReverseComparator(new ComparableComparator()));
         Collections.sort(myList, reversedNaturalOrderBeanComparator);
         ...

- Related Packages
  Package
  Description
  org.apache.commons.beanutils.converters
  Standard implementations of the `Converter` interface that are pre-registered with `ConvertUtils` at startup time.
  org.apache.commons.beanutils.expression
  Contains the `Resolver` interface and implementations.
  org.apache.commons.beanutils.locale
  Locale-aware extensions of the standard beanutils classes.
- Class
  Description
  BaseDynaBeanMapDecorator<K>
  A base class for decorators providing `Map` behavior on `DynaBean`s.
  BasicDynaBean
  Minimal implementation of the `DynaBean` interface.
  BasicDynaClass
  Minimal implementation of the `DynaClass` interface.
  BeanAccessLanguageException
  Thrown to indicate that the Bean Access Language cannot execute query against given bean.
  BeanComparator<T>
  This comparator compares two beans by the specified bean property.
  BeanIntrospector
  Definition of an interface for components that can perform introspection on bean classes.
  BeanMap
  An implementation of Map for JavaBeans which uses introspection to get and put properties in the bean.
  BeanMap.Entry
  Map entry used by `BeanMap`.
  BeanPredicate
  Predicate implementation that applies the given `Predicate` to the result of calling the given property getter.
  BeanPropertyValueChangeClosure
  `Closure` that sets a property.
  BeanPropertyValueEqualsPredicate
  `Predicate` that evaluates a property value against a specified value.
  BeanToPropertyValueTransformer
  `Transformer` that outputs a property value.
  BeanUtils
  Utility methods for populating JavaBeans properties via reflection.
  BeanUtilsBean
  JavaBean property population methods.
  BeanUtilsBean2
  `BeanUtilsBean` implementation that creates a `ConvertUtilsBean2` and delegates conversion to `ConvertUtilsBean.convert(Object, Class)`.
  ConstructorUtils
  Utility reflection methods focused on constructors, modeled after `MethodUtils`.
  ContextClassLoaderLocal<T>
  An instance of this class represents a value that is provided per (thread) context classloader.
  ConversionException
  A ConversionException indicates that a call to `Converter.convert()` has failed to complete successfully.
  Converter
  General purpose data type converter that can be registered and used within the BeanUtils package to manage the conversion of objects from one type to another.
  ConvertingWrapDynaBean
  Implements `DynaBean` to wrap a standard JavaBean instance, so that DynaBean APIs can be used to access its properties, though this implementation allows type conversion to occur when properties are set.
  ConvertUtils
  Utility methods for converting String scalar values to objects of the specified Class, String arrays to arrays of the specified Class.
  ConvertUtilsBean
  Utility methods for converting String scalar values to objects of the specified Class, String arrays to arrays of the specified Class.
  ConvertUtilsBean2
  `ConvertUtilsBean` implementation that delegates `convert()` methods to the new `ConvertUtilsBean.convert(Object, Class)` method.
  DefaultBeanIntrospector
  The default `BeanIntrospector` implementation.
  DynaBean
  A DynaBean is a Java object that supports properties whose names and data types, as well as values, may be dynamically modified.
  DynaBeanMapDecorator
  Deprecated.
  Use `DynaBeanPropertyMapDecorator` instead.
  DynaBeanPropertyMapDecorator
  Decorates a `DynaBean` to provide `Map` behavior.
  DynaClass
  A DynaClass is a simulation of the functionality of `java.lang.Class` for classes implementing the `DynaBean` interface.
  DynaProperty
  The metadata describing an individual property of a DynaBean.
  FluentPropertyBeanIntrospector
  An implementation of the `BeanIntrospector` interface which can detect write methods for properties used in fluent API scenario.
  IntrospectionContext
  A context interface used during introspection for querying and setting property descriptors.
  LazyDynaBean
  DynaBean which automatically adds properties to the `DynaClass` and provides Lazy List and Lazy Map features.
  LazyDynaClass
  DynaClass which implements the `MutableDynaClass` interface.
  LazyDynaList
  Lazy DynaBean List.
  LazyDynaMap
  Provides a light weight `DynaBean` facade to a `Map` with lazy map/list processing.
  MappedPropertyDescriptor
  A MappedPropertyDescriptor describes one mapped property.
  MethodUtils
  Utility reflection methods focused on methods in general rather than properties in particular.
  MutableDynaClass
  A specialized extension to `DynaClass` that allows properties to be added or removed dynamically.
  NestedNullException
  Thrown to indicate that the Bean Access Language cannot execute query against given bean since a nested bean referenced is null.
  PropertyUtils
  Utility methods for using Java Reflection APIs to facilitate generic property getter and setter operations on Java objects.
  PropertyUtilsBean
  Utility methods for using Java Reflection APIs to facilitate generic property getter and setter operations on Java objects.
  ResultSetDynaClass
  Implements `DynaClass` for DynaBeans that wrap the `java.sql.Row` objects of a `java.sql.ResultSet`.
  ResultSetIterator
  Implements `Iterator` returned by the `iterator()` method of `ResultSetDynaClass`.
  RowSetDynaClass
  Implements `DynaClass` to create an in-memory collection of `DynaBean`s representing the results of an SQL query.
  SuppressPropertiesBeanIntrospector
  A specialized `BeanIntrospector` implementation which suppresses some properties.
  WrapDynaBean
  Implements `DynaBean` to wrap a standard JavaBean instance, so that DynaBean APIs can be used to access its properties.
  WrapDynaClass
  Implements `DynaClass` for DynaBeans that wrap standard JavaBean instances.