Reason for the exception java.lang.VerifyError: Ba

Question

The below simple java code sends the java.lang.VerifyError: Bad type on operand stack exception

public class TestJavaCodes {

    int parentData = 0;

    public void init() {
        A ob = new B();
    }

    public static void main(String[] args) {

        TestJavaCodes testJavaCodes = new TestJavaCodes();
        testJavaCodes.init();
    }

    public static class A {
        public A(MyLambdaFunc lambdaFunc) {
        }
    }

    public class B extends A {

        public B() {
            super((data1, type) -> {
                parentData = 1;
            });
        }
    }

    @FunctionalInterface
    public static interface MyLambdaFunc {
        public void onData(String data, int type);
    }
}

If I remove the code

parentData = 1

from B's constructor, the exception won't come.

Can any one tell the reason for this?

Answer 1

Seems that such code should not compile at all. I minimized your code:

public class CompilerBug {
    int var = 0;

    public static void main(String[] args) {
        new CompilerBug().new Inner();
    }

    public class Inner {
        public Inner(Runnable r) {}

        public Inner() {
            this(() -> {
                var = 1;
            });
        }
    }
}

It's compiled without problems by javac 1.8.0.25, 1.8.0.40 and 1.9b57. Every compiled version produces the same output when launching:

Exception in thread "main" java.lang.VerifyError: Bad type on operand stack
Exception Details:
  Location:
    CompilerBug$Inner.<init>(LCompilerBug;)V @3: invokedynamic
  Reason:
    Type uninitializedThis (current frame, stack[2]) is not assignable to 'CompilerBug$Inner'
  Current Frame:
    bci: @3
    flags: { flagThisUninit }
    locals: { uninitializedThis, 'CompilerBug' }
    stack: { uninitializedThis, 'CompilerBug', uninitializedThis }
  Bytecode:
    0000000: 2a2b 2aba 0003 0000 b700 04b1

        at CompilerBug.main(CompilerBug.java:5)

This code is not compiled by ECJ compiler. It reports a compilation error:

----------
1. ERROR in C:\projects\Test\src\CompilerBug.java (at line 12)
    this(() -> {
         ^^^^^
Cannot refer to 'this' nor 'super' while explicitly invoking a constructor
----------
1 problem (1 error)

So it looks like a bug in javac compiler: it should return a compilation error instead (like ECJ).

I did not find similar bug in OpenJDK bug tracker, so submitted a new bug report via webform. If Java folks are reading this, the internal review ID assigned is JI-9021379.

Update: The bug report is accepted (JDK-8129740)

Answer 2

The problem arises because your lambda expression does not reference this or a member of this but a member of the outer this. Had you written class B like

public class B extends A {
    int innerData;
    public B() {
        super((data1, type) -> innerData = 1);
    }
}

the compiler rejected it without any doubts as accessing innerData implies accessing this.

The point about the outer instance is that it is a constant which is even available when the inner instance has not been fully constructed yet. So it’s correct to accept the code but unfortunately the compiler generates code which attempts to access the outer instance via an implicit field of the inner class instance, thus the lambda expression requires an instance of the inner class and attempting to use the not fully constructed inner class instance produces the error.

It can be easily demonstrated that the code can be compiled correctly:

public class B extends A {
    public B() {
        this(TestJavaCodes.this);
    }
    private B(TestJavaCodes outer) {
        super((data1, type) -> outer.parentData = 1);
    }
}

with that small change, the lambda expression refers to the outer instance without accessing the inner instance and no error arises.