方法句柄

public class Foo {
    private static void bar(Object o) {
    }

    private static MethodHandles.Lookup lookup() {
        // Lookup的创建位置，具备Foo类的访问权限
        return MethodHandles.lookup();
    }

    public static void main(String[] args) throws Exception {
        // 获取方法句柄的不同方式
        MethodHandles.Lookup lookup = Foo.lookup();
        Method method = Foo.class.getDeclaredMethod("bar", Object.class);
        MethodHandle mh0 = lookup.unreflect(method);

        MethodType t = MethodType.methodType(void.class, Object.class);
        MethodHandle mh1 = lookup.findStatic(Foo.class, "bar", t);
    }
}

// 严格匹配参数类型
public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable;
// 自动适配参数类型
public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable;

public void test(MethodHandle handle, String s) throws Throwable {
    handle.invokeExact(s);
    handle.invokeExact((Object) s);
}

public void test(java.lang.invoke.MethodHandle, java.lang.String) throws java.lang.Throwable;
  descriptor: (Ljava/lang/invoke/MethodHandle;Ljava/lang/String;)V
  flags: ACC_PUBLIC
  Code:
    stack=2, locals=3, args_size=3
       0: aload_1
       1: aload_2
       // 传入的参数为String，对应的方法描述符为String
       2: invokevirtual     // Method java/lang/invoke/MethodHandle.invokeExact:(Ljava/lang/String;)V
       5: aload_1
       6: aload_2
       // 转化为Object，对应的方法描述符为Object
       7: invokevirtual     // Method java/lang/invoke/MethodHandle.invokeExact:(Ljava/lang/Object;)V
      10: return

// -XX:+UnlockDiagnosticVMOptions -XX:+ShowHiddenFrames -> 打印隐藏的栈信息
// -Djava.lang.invoke.MethodHandle.DUMP_CLASS_FILES=true -> 导出class文件
public class A {
    private static void bar(Object o) {
        new Exception().printStackTrace();
    }

    public static void main(String[] args) throws Throwable {
        MethodHandles.Lookup lookup = MethodHandles.lookup();
        MethodType methodType = MethodType.methodType(void.class, Object.class);
        MethodHandle methodHandle = lookup.findStatic(A.class, "bar", methodType);
        methodHandle.invokeExact(new Object());
    }
}

java.lang.Exception
	at me.zhongmingmao.basic.method_handle.A.bar(A.java:11)
	at java.lang.invoke.LambdaForm$DMH001/746292446.invokeStatic_001_L_V(LambdaForm$DMH001:1000010)
	at java.lang.invoke.LambdaForm$MH012/468121027.invokeExact_000_MT(LambdaForm$MH012:1000016)
	at me.zhongmingmao.basic.method_handle.A.main(A.java:18)

$ javap -v -p -c LambdaForm\$MH012
static void invokeExact_000_MT(java.lang.Object, java.lang.Object, java.lang.Object);
   descriptor: (Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)V
   flags: ACC_STATIC
   Code:
     stack=2, locals=3, args_size=3
        0: aload_0
        1: aload_2
        // 检验参数类型
        2: invokestatic     // Method java/lang/invoke/Invokers.checkExactType:(Ljava/lang/Object;Ljava/lang/Object;)V
        5: aload_0
        // -Djava.lang.invoke.MethodHandle.CUSTOMIZE_THRESHOLD=127
        // checkCustomized：是否为为该方法句柄生成一个特有的适配器
        6: invokestatic     // Method java/lang/invoke/Invokers.checkCustomized:(Ljava/lang/Object;)V
        9: aload_0
       10: checkcast        // class java/lang/invoke/MethodHandle
       13: dup
       14: astore_0
       15: aload_1
       // JVM同样会对invokeBasic做特殊处理，将会调用至方法句柄本身所持有的适配器中，该适配器同样是LambdaForm
       16: invokevirtual    // Method java/lang/invoke/MethodHandle.invokeBasic:(Ljava/lang/Object;)V
       19: return

$ javap -v -p -c LambdaForm\$DMH001
static void invokeStatic_001_L_V(java.lang.Object, java.lang.Object);
  descriptor: (Ljava/lang/Object;Ljava/lang/Object;)V
  flags: ACC_STATIC
  Code:
    stack=2, locals=3, args_size=2
       0: aload_0
       1: invokestatic  #16                 // Method java/lang/invoke/DirectMethodHandle.internalMemberName:(Ljava/lang/Object;)Ljava/lang/Object;
       4: astore_2
       5: aload_1
       6: aload_2
       7: checkcast     #18                 // class java/lang/invoke/MemberName
       // JVM会对linkToStatic调用做特殊处理，
       // 将根据传入的MemberName参数所存储的方法地址或者方法表索引，直接跳转到目标方法
      10: invokestatic  #24                 // Method java/lang/invoke/MethodHandle.linkToStatic:(Ljava/lang/Object;Ljava/lang/invoke/MemberName;)V
      13: return