Java中的註解(Annotation)是一個很神奇的東西，特別現在有很多Android庫都是使用註解的方式來實現的。 我們並不討論那些在執行時（Runtime）通過反射機制執行處理的註解，而是討論在編譯時（Compile time）處理的註解。下面便入手學習下Java註解處理器。

簡單實踐： 1.建立註解

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.CLASS)
public @interface MethodProcessor {
    String name() default "Method";
}

接下來就可以使用了

@MethodProcessor(name = "Method")
public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

2.註解處理器 上面只是講如何運用，其實內部是如何生成的暫時還不知道，往下看。

//@AutoService(Processor.class) 這個有木有？這是一個註解處理器，是Google開發的，
//用來生成META-INF/services/javax.annotation.processing.Processor檔案的。
//引入方式     compile 'com.google.auto.service:auto-service:1.0-rc2'
@AutoService(Processor.class)
public class MyProcessor extends AbstractProcessor {

    private Types mTypeUtils;
    private Elements mElementUtils;
    private Filer mFiler;
    private Messager mMessager;

    @Override
    public Set<String> getSupportedAnnotationTypes() {
        Set<String> annotations = new LinkedHashSet<>();
        //把我們自己定義的註解新增進去
        annotations.add(MethodProcessor.class.getCanonicalName());
        return annotations;
    }

    @Override
    public SourceVersion getSupportedSourceVersion() {
        return SourceVersion.latestSupported();
    }

    @Override
    public synchronized void init(ProcessingEnvironment processingEnv) {
        super.init(processingEnv);

        mTypeUtils = processingEnv.getTypeUtils();
        mElementUtils = processingEnv.getElementUtils();
        mFiler = processingEnv.getFiler();
        mMessager = processingEnv.getMessager();
    }

    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
    	// // 遍歷所有被註解了的元素
        for (Element annotatedElement : roundEnv.getElementsAnnotatedWith(MethodProcessor.class)) {
            if (annotatedElement.getKind() != ElementKind.CLASS) {
                error(annotatedElement, "Only classes can be annotated with @%s", MethodProcessor.class.getSimpleName());
                return true;
            }
            // //解析，並生成程式碼
            analysisAnnotated(annotatedElement);
        }
        return false;
    }

    private void error(Element e, String msg, Object... args) {
        mMessager.printMessage(Diagnostic.Kind.ERROR, String.format(msg, args), e);
    }

    private static final String SUFFIX = "Test";
    private static final String packageName = "com.company.processtest";
    private static final String retStr = "";

    private void analysisAnnotated(Element classElement) {
        MethodProcessor annotation = classElement.getAnnotation(MethodProcessor.class);
        String name = annotation.name();

        String newClassName = name + SUFFIX;

        StringBuilder builder = new StringBuilder()
                .append("package " + packageName + ";\n\n")
                .append("public class ")
                .append(newClassName)
                .append(" {\n\n") // open class
                .append("\tpublic String getMessage() {\n") // open method
                .append("\t\treturn \"");

        // this is appending to the return statement
        builder.append(retStr).append(" !\\n");

        builder.append("\";\n") // end return
                .append("\t}\n") // close method
                .append("}\n"); // close class

        try { // write the file
            JavaFileObject source = mFiler.createSourceFile(packageName + "." + newClassName);
            Writer writer = source.openWriter();
            writer.write(builder.toString());
            writer.flush();
            writer.close();
        } catch (IOException e) {
        }
    }
}
 

最後build一下工程，生成最終的java檔案 檔案位置ProcessTest\app\build\generated\source\apt\debug\com\company\processtest\MethodTest.java

package com.company.processtest;

public class MethodTest {

	public String getMessage() {
		return "this is just a test !\n";
	}
}

這樣，就可以使用這個類了

@MethodProcessor(name = "Method")
public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        MethodTest test = new MethodTest();
        test.getMessage();
    }
}
 

注意：註解類和AbstractProcessor必須要新建一個java庫。

AbstractProcessor中生成java類，可以使用JavaPoet開源庫進行編寫，提升效率