com.google.auto.value.extension

Class AutoValueExtension

java.lang.Object

com.google.auto.value.extension.AutoValueExtension

Direct Known Subclasses:

MemoizeExtension

public abstract class AutoValueExtension
extends java.lang.Object

An AutoValueExtension allows for extra functionality to be created during the generation of an AutoValue class.

Extensions are discovered at compile time using the ServiceLoader APIs, allowing them to run without any additional annotations. To be found by ServiceLoader, an extension class must be public with a public no-arg constructor, and its fully-qualified name must appear in a file called META-INF/services/com.google.auto.value.extension.AutoValueExtension in a jar that is on the compiler's -classpath or -processorpath.

When the AutoValue processor runs for a class Foo, it will ask each Extension whether it is applicable. Suppose two Extensions reply that they are. Then the processor will generate the AutoValue logic in a direct subclass of Foo, and it will ask the first Extension to generate a subclass of that, and the second Extension to generate a subclass of the subclass. So we might have this hierarchy:

 @AutoValue abstract class Foo {...}                          // the hand-written class
 abstract class $$AutoValue_Foo extends Foo {...}             // generated by AutoValue processor
 abstract class $AutoValue_Foo extends $$AutoValue_Foo {...}  // generated by first Extension
 final class AutoValue_Foo extends $AutoValue_Foo {...}       // generated by second Extension
 

(The exact naming scheme illustrated here is not fixed and should not be relied on.)

If an Extension needs its generated class to be the final class in the inheritance hierarchy, its mustBeFinal(Context) method returns true. Only one Extension can return true for a given context. Only generated classes that will be the final class in the inheritance hierarchy can be declared final. All others should be declared abstract.

The first generated class in the hierarchy will always be the one generated by the AutoValue processor and the last one will always be the one generated by the Extension that mustBeFinal, if any. Other than that, the order of the classes in the hierarchy is unspecified. The * last class in the hierarchy is AutoValue_Foo and that is the one that the Foo class will reference, for example with new AutoValue_Foo(...).

Each Extension must also be sure to generate a constructor with arguments corresponding to all properties in AutoValueExtension.Context.properties(), in order, and to call the superclass constructor with the same arguments. This constructor must have at least package visibility.

Because the class generated by the AutoValue processor is at the top of the generated hierarchy, Extensions can override its methods, for example hashCode(), toString(), or the implementations of the various bar() property methods.

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static interface	AutoValueExtension.Context The context of the generation cycle.
static class	AutoValueExtension.IncrementalExtensionType Indicates to an annotation processor environment supporting incremental annotation processing (currently a feature specific to Gradle starting with version 4.8) the incremental type of an Extension.

Constructor Summary

Constructors

Constructor and Description
AutoValueExtension()

Method Summary

Modifier and Type	Method and Description
boolean	applicable(AutoValueExtension.Context context) Determines whether this Extension applies to the given context.
java.util.Set<javax.lang.model.element.ExecutableElement>	consumeMethods(AutoValueExtension.Context context) Returns a possible empty set of abstract methods that this Extension intends to implement.
java.util.Set<java.lang.String>	consumeProperties(AutoValueExtension.Context context) Returns a possibly empty set of property names that this Extension intends to implement.
abstract java.lang.String	generateClass(AutoValueExtension.Context context, java.lang.String className, java.lang.String classToExtend, boolean isFinal) Returns the generated source code of the class named className to extend classToExtend, or null if this extension does not generate a class in the hierarchy.
AutoValueExtension.IncrementalExtensionType	incrementalType(javax.annotation.processing.ProcessingEnvironment processingEnvironment) Determines the incremental type of this Extension.
boolean	mustBeFinal(AutoValueExtension.Context context) Denotes that the class generated by this Extension must be the final class in the inheritance hierarchy.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

AutoValueExtension

public AutoValueExtension()

Method Detail

incrementalType

public AutoValueExtension.IncrementalExtensionType incrementalType(javax.annotation.processing.ProcessingEnvironment processingEnvironment)

Determines the incremental type of this Extension.

The ProcessingEnvironment can be used, among other things, to obtain the processor options, using ProcessingEnvironment.getOptions().

The actual incremental type of the AutoValue processor as a whole will be the loosest incremental types of the Extensions present in the annotation processor path. The default returned value is AutoValueExtension.IncrementalExtensionType.UNKNOWN, which will disable incremental annotation processing entirely.

applicable

public boolean applicable(AutoValueExtension.Context context)

Determines whether this Extension applies to the given context.

Parameters:

context - The Context of the code generation for this class.

Returns:

true if this Extension should be applied in the given context. If an Extension returns false for a given class, it will not be called again during the processing of that class.

mustBeFinal

public boolean mustBeFinal(AutoValueExtension.Context context)

Denotes that the class generated by this Extension must be the final class in the inheritance hierarchy. Only one Extension may be the final class, so this should be used sparingly.

Parameters:

context - the Context of the code generation for this class.

consumeProperties

public java.util.Set<java.lang.String> consumeProperties(AutoValueExtension.Context context)

Returns a possibly empty set of property names that this Extension intends to implement. This will prevent AutoValue from generating an implementation, and remove the supplied properties from builders, constructors, toString, equals, and hashCode. The default set returned by this method is empty.

Each returned string must be one of the property names in AutoValueExtension.Context.properties().

Returning a property name from this method is equivalent to returning the property's getter method from consumeMethods(com.google.auto.value.extension.AutoValueExtension.Context).

For example, Android's Parcelable interface includes a method int describeContents(). Since this is an abstract method with no parameters, by default AutoValue will consider that it defines an int property called describeContents. If an @AutoValue class implements Parcelable and does not provide an implementation of this method, by default its implementation will include describeContents in builders, constructors, and so on. But an AutoValueExtension that understands Parcelable can instead provide a useful implementation and return a set containing "describeContents". Then describeContents will be omitted from builders and the rest.

Parameters:

context - the Context of the code generation for this class.

consumeMethods

public java.util.Set<javax.lang.model.element.ExecutableElement> consumeMethods(AutoValueExtension.Context context)

Returns a possible empty set of abstract methods that this Extension intends to implement. This will prevent AutoValue from generating an implementation, in cases where it would have, and it will also avoid warnings about abstract methods that AutoValue doesn't expect. The default set returned by this method is empty.

Each returned method must be one of the abstract methods in AutoValueExtension.Context.abstractMethods().

For example, Android's Parcelable interface includes a method void writeToParcel(Parcel, int). Normally AutoValue would not know what to do with that abstract method. But an AutoValueExtension that understands Parcelable can provide a useful implementation and return the writeToParcel method here. That will prevent a warning about the method from AutoValue.

Parameters:

context - the Context of the code generation for this class.

generateClass

public abstract java.lang.String generateClass(AutoValueExtension.Context context,
                                               java.lang.String className,
                                               java.lang.String classToExtend,
                                               boolean isFinal)

Returns the generated source code of the class named className to extend classToExtend, or null if this extension does not generate a class in the hierarchy. If there is a generated class, it should be final if isFinal is true; otherwise it should be abstract. The returned string should be a complete Java class definition of the class className in the package context.packageName().

The returned string will typically look like this:

 package <package>;
 ...
 <finalOrAbstract> class <className> extends <classToExtend> {...}
 

Here, <package> is AutoValueExtension.Context.packageName(); <finalOrAbstract> is the keyword final if isFinal is true or abstract otherwise; and <className> and <classToExtend> are the values of this method's parameters of the same name.

Parameters:

context - The AutoValueExtension.Context of the code generation for this class.

className - The simple name of the resulting class. The returned code will be written to a file named accordingly.

classToExtend - The simple name of the direct parent of the generated class. This could be the AutoValue generated class, or a class generated as the result of another Extension.

isFinal - True if this class is the last class in the chain, meaning it should be marked as final. Otherwise it should be marked as abstract.

Returns:

The source code of the generated class, or null if this extension does not generate a class in the hierarchy.