What is the reason why “synchronized” is not allowed in Java 8 interface methods?

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In Java 8, I can easily write: 

interface Interface1 {
    default void method1() {
        synchronized (this) {
            // Something
        }
    }

    static void method2() {
        synchronized (Interface1.class) {
            // Something
        }
    }
}

I will get the full synchronisation semantics that I can use also in classes. I cannot, however, use the synchronized modifier on method declarations:

interface Interface2 {
    default synchronized void method1() {
        //  ^^^^^^^^^^^^ Modifier 'synchronized' not allowed here
    }

    static synchronized void method2() {
        // ^^^^^^^^^^^^ Modifier 'synchronized' not allowed here
    }
}

Now, one can argue that the two interfaces behave the same way except that Interface2 establishes a contract on method1() and on method2(), which is a bit stronger than what Interface1 does. Of course, we might also argue that default implementations should not make any assumptions about concrete implementation state, or that such a keyword simply wouldn’t pull its weight.

Question:

What is the reason why the JSR-335 expert group decided not to support synchronized on interface methods?

2 Answers
2

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