Java和Javap：如何确定哪个对象接收了调用虚拟

Question

I am looking through the output of Javap. For example:

This code

    final Foo foo = new Foo(1,2);
    ... 
    new Callable<Integer>()
    {
        @Override
        public Integer call() throws Exception
        {
            return foo.doSomething();
        }

Generates:

        jvmOperations": [{
            "byteOffset": 0,
            "constantPoolIndex": null,
            "opCode": 42,
            "opCodeName": "aload_0",
            "type": null,
            "tagName": null,
            "tagValue": null
        }, {
            "byteOffset": 1,
            "constantPoolIndex": null,
            "opCode": 180,
            "opCodeName": "getfield",
            "type": null,
            "tagName": "Field",
            "tagValue": "val$foo:Lcom/example/graph/demo/Foo;"
        }, {
            "byteOffset": 4,
            "constantPoolIndex": null,
            "opCode": 182,
            "opCodeName": "invokevirtual",
            "type": null,
            "tagName": "Method",
            "tagValue": "com/example/graph/demo/Foo.doSomething:()Ljava/lang/Integer;"
        }, {
            "byteOffset": 7,
            "constantPoolIndex": null,
            "opCode": 176,
            "opCodeName": "areturn",
            "type": null,
            "tagName": null,
            "tagValue": null
        }]

So I see that the object is identified in this case by val$foo . And in the class metadata

    "classMetaData": {
        "classId": "com/example/Main$1.class",
        "sourceName": "Main.java",
        "isInterface": false,
        "isClass": true,
        "accessModifiers": ["final"],
        "superClassName": "java/lang/Object",
        "implementedInterfaces": ["java/util/concurrent/Callable"],
        "jreTargetVersion": "51.0",
        "fields": ["val$foo"],
        "fieldModifiers": {
            "val$foo": ["final"]
        },
        "methodInformationMap": {},
        "interface": false,
        "class": true
    }, 

But now I want to find out more about the original object foo . For example, I know it has this data in one of its fields:

        {
            "byteOffset": 37,
            "constantPoolIndex": null,
            "opCode": 18,
            "opCodeName": "ldc",
            "type": null,
            "tagName": "String",
            "tagValue": "NODE-1"
        }, 

How does the JVM know what val$foo points to?

Answer 1

You need a little more context to track the value the JVM stores for foo .

Assuming foo is a local variable

1. new Foo(1,2); is invoked
2. The result, the value of the reference to the instance, is copied and stored in the local variable foo
3. ...stuff...
4. The anonymous class constructor is invoked to create a new instance
5. As part of its constructor, a copy of the value of the local variable foo is retrieved and pushed on the stack
6. That value is popped from the stack and assigned to this val$foo field of the anonymous class (this is closing over that variable)
7. ...stuff...
8. When foo.something() is invoked, the JVM retrieves the value of the field val$foo of the anonymous class instance
9. The JVM dereferences that value to get the object and invokes the method on it