A strategy (or policy) definition that is used to decide whether or not a field or class should be serialized or deserialized as part of the JSON output/input.

The following are a few examples that shows how you can use this exclusion mechanism.

Exclude fields and objects based on a particular class type:

private static class SpecificClassExclusionStrategy implements ExclusionStrategy {
  private final Class<?> excludedThisClass;

  public SpecificClassExclusionStrategy(Class<?> excludedThisClass) {
    this.excludedThisClass = excludedThisClass;
  }

  public boolean shouldSkipClass(Class<?> clazz) {
    return excludedThisClass.equals(clazz);
  }

  public boolean shouldSkipField(FieldAttributes f) {
    return excludedThisClass.equals(f.getDeclaredClass());
  }
}

Excludes fields and objects based on a particular annotation:

public @interface FooAnnotation {
  // some implementation here
}

// Excludes any field (or class) that is tagged with an "@FooAnnotation"
private static class FooAnnotationExclusionStrategy implements ExclusionStrategy {
  public boolean shouldSkipClass(Class<?> clazz) {
    return clazz.getAnnotation(FooAnnotation.class) != null;
  }

  public boolean shouldSkipField(FieldAttributes f) {
    return f.getAnnotation(FooAnnotation.class) != null;
  }
}

Now if you want to configure Gson to use a user defined exclusion strategy, then the GsonBuilder is required. The following is an example of how you can use the GsonBuilder to configure Gson to use one of the above samples:

ExclusionStrategy excludeStrings = new UserDefinedExclusionStrategy(String.class);
Gson gson = new GsonBuilder()
    .setExclusionStrategies(excludeStrings)
    .create();

For certain model classes, you may only want to serialize a field, but exclude it for deserialization. To do that, you can write an ExclusionStrategy as per normal; however, you would register it with the GsonBuilder#addDeserializationExclusionStrategy(ExclusionStrategy) method. For example:

ExclusionStrategy excludeStrings = new UserDefinedExclusionStrategy(String.class);
Gson gson = new GsonBuilder()
    .addDeserializationExclusionStrategy(excludeStrings)
    .create();