关于私有成员访问的JDK 7中的javac行为更改

Question

This code compiles OK using javac JDK version 1.6.0_33-b03-424, but doesn't compile using javac JDK version 1.7.0_06.

public class Test {
    private final int i = 0;

    void test(Object o) {
        if (getClass().isInstance(o)) {
            System.out.println(getClass().cast(o).i);
        }
    }
}

javac output is:

Test.java:6: error: i in Test is defined in an inaccessible class or interface
        System.out.println(getClass().cast(o).i);
                                             ^
1 error

Changing the code to store the result of getClass.cast() in a temporary variable allows the program to compile without error.

This is easy to work around, but I can't find any rationale for this change in the JLS 7, or any mention of a change like this in the JDK 7 release notes. There is a mention of an access change regarding private members of type parameters to a generic, but that doesn't apply here.

Is this a regression in javac? Is it now enforcing a restriction that it wasn't enforcing before?

Answer 1

Well, I'm puzzled by this and the only explanation I can adventure is the conjunction of two things.

1_ getClass() docs say the following:

The actual result type is Class<? extends |X|> Class<? extends |X|> where |X| is the erasure of the static type of the expression on which getClass is called.

2_ One of the incompatibilities introduced in Java 7 is Compiler No Longer Allows Access to Private Members of Type Variables .

So, the compiler is unsure it the cast is made to the base class or a subclass and it blocks accesing a private member, since if the cast were to be assigned to a subclass it would be illegal even if defined in the original parent class, as shown in the following example:

class BaseTest {
    private final int i = 1;

    void test(Object o) {
        if (getClass().isInstance(o)) {                
            TestAccess to = TestAccess.class.cast(o);
            //System.out.println(to.i);  // ERROR: i has private access in BaseTest
        }
    }
}

class TestAccess extends BaseTest{}

So, I guess it's one more of Java quirks due to rules that make more sense in more complex examples.