如何使用java.util.LinkedHashMap $ EntryIterator类型访问对象值

Question

I need to access the value for the field called this$0 of object type java.util.LinkedHashMap$EntryIterator type, see the image below.

the object

Iterator<Entry<String, JsonNode>> ite = data.getFields();

as you can see, I have a object "ite" and I need to access the value of field "this$0" , how to get it?

Answer 1

This field is added by compiler to store in inner class reference to its outer class (when inner class is not static). Thanks to this field inner class instance has access to members (even private ones) of outer class instance.

Value of this field is set by outerClassInstance.new InnerClass() so this$0 in InnerClass instance will contain same value as outerClassInstance reference.

You can't use this field directly in code like

class Outer{
    class Inner{
        void test(){
            System.out.println(this$0);//this will not compile
        }
    }
}

but you can access it via reflection. Code of method which will return value of this field could look like

public static Object getOuterInstance(Object inner)
        throws NoSuchFieldException, SecurityException,
        IllegalArgumentException, IllegalAccessException {

    Class<?> clazz = inner.getClass();
    Field f = clazz.getDeclaredField("this$0");
    f.setAccessible(true);//in case of class placed in different package

    return f.get(inner);
}

Example:

class Outer {
    private String name;
    public Outer(String name) { this.name = name; }
    public String getName() { return name; }

    class Inner{}
}

class Demo {

    public static void main(String[] args) throws Exception {

        Outer o1 = new Outer("foo");
        Outer o2 = new Outer("bar");

        Inner i1 = o1.new Inner();
        Inner i2 = o2.new Inner();

        System.out.println(((Outer) getOuterInstance(i1)).getName());
        System.out.println(((Outer) getOuterInstance(i2)).getName());

    }

    public static Object getOuterInstance(Object inner)
            throws NoSuchFieldException, SecurityException,
            IllegalArgumentException, IllegalAccessException {

        Class<?> clazz = inner.getClass();
        Field f = clazz.getDeclaredField("this$0");
        f.setAccessible(true);

        return f.get(inner);
    }

}

Output:

foo
bar

which means that we did in fact get correct instances.

In case of your example you will have to:

1. iterate over each element in your ite iterator
2. use this method on each element from iterator
3. cast result of method to Map or if you need more precise type LinkedHashMap .