MethodHandle Lookup facility

Question

As far as JavaDoc states MethodHandles.lookup() returns facility that have ability to access the same method/functions/constructor as the caller of this function. Specifically, if the caller can access some private data, so as this MethodHandles.Lookup facility. Code below demonstrates that this is false. Where I get it wrong?

public class MethodHandlerAccessTest  {

        private static class NestedClass {
            private static void foo(){}
        }

        @Test
        public void testPrivateAccess() throws Throwable {
            NestedClass.foo();  //compiles and executes perfectly
            MethodType type = MethodType.methodType(void.class);
            MethodHandles.Lookup lookup = MethodHandles.lookup();
            MethodHandle mh = lookup.findStatic(NestedClass.class, "foo", type);
        }

 }

Edit:

This is what I get:

java.lang.IllegalAccessException: member is private: MethodHandlerAccessTest$NestedClass.foo()void, from MethodHandlerAccessTest at java.lang.invoke.MemberName.makeAccessException(MemberName.java:507) at java.lang.invoke.MethodHandles$Lookup.checkAccess(MethodHandles.java:1182) at java.lang.invoke.MethodHandles$Lookup.checkMethod(MethodHandles.java:1162) at java.lang.invoke.MethodHandles$Lookup.accessStatic(MethodHandles.java:591) at java.lang.invoke.MethodHandles$Lookup.findStatic(MethodHandles.java:587) at MethodHandlerAccessTest.testPrivateAccess(MethodHandlerAccessTest.java:19) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:601) at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47) at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12) at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44) at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17) at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50) at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238) at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63) at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236) at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53) at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229) at org.junit.runners.ParentRunner.run(ParentRunner.java:309) at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50) at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

Answer 1

The problem is that your test method doesn't really call NestedClass.foo(). This line:

NestedClass.foo();

... is actually transformed into a call to a synthetic method which is generated in foo, like this:

NestedClass.access$000();

Where access$000 looks like this:

// Note package access
static void access$000() {
    foo();
}

You can validate this by using javap -c to look at the actual bytecode.

At the JVM level, your outer class doesn't have access to foo(). The Java compiler just synthesizes access to it by creating access$000 and calling it from your outer class whenever the source code calls foo().

At execution time, the reflection libraries don't do the same thing, hence your error.