In-depth analysis of annotation processing skills in Java development
Annotations are a very important feature in Java development. They can add metadata and configuration information to the code. Provides a more flexible programming method. In Java development, we often encounter scenarios where annotations need to be processed. This article will provide an in-depth analysis of annotation processing techniques in Java development.
@
symbol before the element. It can be used in various places such as classes, methods, variables, etc. . When defining annotations, you need to use the@interface
keyword to declare them. For example:@Target(ElementType.METHOD) @Retention(RetentionPolicy.RUNTIME) public @interface MyAnnotation { String value() default ""; int size() default 0; }
The above code defines an annotation namedMyAnnotation
, which has two attributesvalue
andsize
, And all have default values. It can be used like this:
@MyAnnotation(value = "hello", size = 10) public void myMethod() { // ... }
javax.annotation.processing.AbstractProcessor
abstract class.The annotation processor can be used to generate code, check the legality of the code, generate documentation, perform code optimization, etc. Here is an example of a simple annotation processor:
@SupportedAnnotationTypes("com.example.MyAnnotation") @SupportedSourceVersion(SourceVersion.RELEASE_8) public class MyAnnotationProcessor extends AbstractProcessor { @Override public boolean process(Set extends TypeElement> annotations, RoundEnvironment roundEnv) { for (TypeElement annotation : annotations) { for (Element annotatedElement : roundEnv.getElementsAnnotatedWith(annotation)) { // 处理注解信息 // ... } } return true; } }
In the above code, we specify the annotation types to be processed through the@SupportedAnnotationTypes
annotation, and through the@SupportedSourceVersion
Annotation specifies supported Java versions. In theprocess
method, we can get the annotation information that needs to be processed through theannotations
parameter, and in theroundEnv.getElementsAnnotatedWith
method we can get the annotated elements. .
Using APT for annotation processing requires the following steps:
1) Write annotation definition;
2) Write annotation processor;
3) Write META-INF/services/javax .annotation.processing.Processor file, which contains the fully qualified name of the annotation processor;
4) When compiling Java files, specify the annotation processor by using the-processor
parameter.
For example, we can use APT to automatically generate code to implement serialization and deserialization of objects. First, write a@Serializable
annotation:
@Target(ElementType.TYPE) @Retention(RetentionPolicy.SOURCE) public @interface Serializable { }
Then, write an annotation processorSerializableProcessor
, by scanning the annotation marked by@Serializable
class, and use StringWriter to write the class information into a Java file.
Finally, when compiling Java files, use the following command to run APT:
javac -processor com.example.SerializableProcessor yourfile.java
To use the AutoService library, you only need to add the@AutoService
annotation to the annotation processor class. For example:
@AutoService(Processor.class) public class MyAnnotationProcessor extends AbstractProcessor { // ... }
Then, when compiling Java files, you still need to use APT to run the annotation processor.
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