如何使用 ASM 扫描字段和方法注释？

Question

使用ASM 5.0.4，我试图找到具有特定注释的类的字段和方法。我喜欢避免加载类而不担心依赖关系。 到目前为止，我无法弄清楚如何使用这种方法获取字段/方法注释：

class AnnotationScanner extends ClassVisitor{

    public AnnotationVisitor visitAnnotation(String desc, boolean visible){
        System.out.println("visitAnnotation: desc="+desc+" visible="+visible);
        return super.visitAnnotation(desc,visible);
    }

    public void visitAttribute(Attribute attr){
        System.out.println("visitAttribute: attr="+attr);
        super.visitAttribute(attr);
    }

    public FieldVisitor visitField(int access, String name, String desc, String signature, Object value){
        System.out.println("visitField: access="+access+" name="+name+" desc="+desc+" signature="+signature+" value="+value);
        return super.visitField(access,name,desc,signature,value);
    }

    public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions){
        System.out.println("visitMethod: access="+access+" name="+name+" desc="+desc+" signature="+signature+" exceptions="+exceptions);
        return super.visitMethod(access,name,desc,signature,exceptions);
    }

    public static void main(String[] args) throws Exception{
        for (String arg : args){
            FileInputStream in = new FileInputStream(new File(arg));
            ClassReader cr = new ClassReader(in);
            cr.accept(new AnnotationScanner(Opcodes.ASM4),0);
        }
    }
}

整个示例项目位于GitHub。

针对一个简单的类运行它

@MyClass(name="annotation scanner")
public class Scannee{
    @MyField(name="a string field") public String aStringField;
    @MyMethod(name="a method")      public void aMethod(){}
}

给予

visitAnnotation: desc=Lorg/springdot/sandbox/asm/MyClass; visible=true
visitField: access=1 name=aStringField desc=Ljava/lang/String; signature=null value=null
visitMethod: access=1 name=<init> desc=()V signature=null exceptions=null
visitMethod: access=1 name=aMethod desc=()V signature=null exceptions=null

但不是字段和方法的注解。

如何获取字段和方法注释？

Answer 1

The example 没有显示如何访问 @MyMethod 注释 (name="a method") 的 name 值。要同时读取 a method 值，您必须再添加两个 Visitor：

static class MethodAnnotationScanner extends MethodVisitor {
        MethodAnnotationScanner() { 
          super(Opcodes.ASM8); 
    }

    @Override
    public AnnotationVisitor visitAnnotation(String desc, boolean visible) {
        System.out.println("visitMethodAnnotation: type= " + desc);
        return new MyAnnotationVisitor();
    }
}

static class MethodAnnotationValueScanner extends AnnotationVisitor {
    MethodAnnotationValueScanner() { 
        super(Opcodes.ASM8); 
    }

    @Override
    public void visit(String name, Object value) {
        System.out.println("visitMethodAnnotationValue: " + name + " = " + value);
        super.visit(name, value);
    }
}

并通过将MethodAnnotationScanner 重新插入visitMethod 来插入它们：

@Override
public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
    System.out.println("visitMethod: access="+access+" name="+name+" desc="+desc+" signature="+signature+" exceptions="+exceptions);
    return new MethodAnnotationScanner();
}

您可以在此处查看完整更新的示例：how-to-read-a-java-class-method-annotation-value-with-asm

Answer 2

您需要继承 FieldVisitor 和 MethodVisitor 并覆盖方法 visitAnnotation，类似于您的 ClassVisitor 子类。例如，

class FieldAnnotationScanner extends FieldVisitor {
    public FieldAnnotationScanner() {
        super(Opcodes.ASM5);
    }

    @Override
    public AnnotationVisitor visitAnnotation(String desc, boolean visible) {
        System.out.println("visitAnnotation: desc="+desc+" visible="+visible);
        return super.visitAnnotation(desc, visible);
    }
}

class MethodAnnotationScanner extends MethodVisitor {
    public MethodAnnotationScanner() {
        super(Opcodes.ASM5);
    }

    @Override
    public AnnotationVisitor visitAnnotation(String desc, boolean visible) {
        System.out.println("visitAnnotation: desc="+desc+" visible="+visible);
        return super.visitAnnotation(desc, visible);
    }
}

然后在visitField 和visitMethod 中将它们连接到您的AnnotationScanner。例如，将您的代码更改为

@Override
public FieldVisitor visitField(int access, String name, String desc, String signature, Object value){
    System.out.println("visitField: access="+access+" name="+name+" desc="+desc+" signature="+signature+" value="+value);
    return new FieldAnnotationScanner();
}

@Override
public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions){
    System.out.println("visitMethod: access="+access+" name="+name+" desc="+desc+" signature="+signature+" exceptions="+exceptions);
    return new MethodAnnotationScanner();
}