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## org.apache.logging.log4j.core.lookup
Interface StrLookup

All Known Implementing Classes:
  AbstractLookup, ContextMapLookup, DateLookup, EnvironmentLookup, Interpolator, JavaLookup, JmxRuntimeInputArgumentsLookup, JndiLookup, MapLookup, ResourceBundleLookup, StructuredDataLookup, SystemPropertiesLookup

    public interface StrLookup

Lookup a String key to a String value.

This class represents the simplest form of a string to string map. It has a benefit over a map in that it can create the result on demand based on the key.

This class comes complete with various factory methods. If these do not suffice, you can subclass and implement your own matcher.

For example, it would be possible to implement a lookup that used the key as a primary key, and looked up the value on demand from the database

Field Summary
`static String` | `CATEGORY` Main plugin category for StrLookup plugins.

Method Summary
`String` | `lookup(LogEvent event, String key)` Looks up a String key to a String value possibly using the current LogEvent.
`String` | `lookup(String key)` Looks up a String key to a String value.

Field Detail

### CATEGORY

    static final String CATEGORY

  Main plugin category for StrLookup plugins.

  Since:
    2.1
  See Also:
    Constant Field Values

Method Detail

### lookup

    String lookup(String key)

  Looks up a String key to a String value.

  The internal implementation may use any mechanism to return the value. The simplest implementation is to use a Map. However, virtually any implementation is possible.

  For example, it would be possible to implement a lookup that used the key as a primary key, and looked up the value on demand from the database Or, a numeric based implementation could be created that treats the key as an integer, increments the value and return the result as a string - converting 1 to 2, 15 to 16 etc.

  This method always returns a String, regardless of the underlying data, by converting it as necessary. For example:

       Map<String, Object> map = new HashMap<String, Object>();
       map.put("number", new Integer(2));
       assertEquals("2", StrLookup.mapLookup(map).lookup("number"));

  Parameters:
    `key` - the key to be looked up, may be null
  Returns:
    the matching value, null if no match

### lookup

    String lookup(LogEvent event,
                  String key)

  Looks up a String key to a String value possibly using the current LogEvent.

  The internal implementation may use any mechanism to return the value. The simplest implementation is to use a Map. However, virtually any implementation is possible.

  For example, it would be possible to implement a lookup that used the key as a primary key, and looked up the value on demand from the database Or, a numeric based implementation could be created that treats the key as an integer, increments the value and return the result as a string - converting 1 to 2, 15 to 16 etc.

  This method always returns a String, regardless of the underlying data, by converting it as necessary. For example:

       Map<String, Object> map = new HashMap<String, Object>();
       map.put("number", new Integer(2));
       assertEquals("2", StrLookup.mapLookup(map).lookup("number"));

  Parameters:
    `event` - The current LogEvent.
    `key` - the key to be looked up, may be null
  Returns:
    the matching value, null if no match

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