java.lang.ClassCastException with null message and cause

Question

I work on a project using Java 8

We have a negative test similar to this:

public Integer g(Object data)
{
    try
    {
        Double d = (Double)data;
    }
    catch(ClassCastException ex)
    {
        if( ex.getMessage() == null )
        {
            return 1;
        }
    }
    return 0;
}

@Test
public void h()
{
    Integer count = 0;
    for( Integer idx = 0; idx < 100000; idx++ )
    {
        // The test
        count += g(0.7312345f);
    }
    System.out.println("Total ClassCastException's with null message: "+count);
}

The negative test expects the exception java.lang.ClassCastException with message "java.lang.Float cannot be cast to java.lang.Double" and it sometimes gets null message

I tried to debug it in eclipse but somehow when attached to debugger the exception and message were as expected all the time

Answer 1

Running the complete example as given by AxelH in OpenJDK 8 revealed my suspicions to be correct.

Copying, so it doesn't go away (which he said he will delete):

public class Main{

    int cnt = 0, cntNull = 0;
    public static void main(String[] args) { 
        new Main().test();
    }

    public void test(){
        for( Integer idx = 0; idx < 200000; idx++ )
        {
            loseType(0.45642f, idx);
        }
        System.out.println(cnt + " error");
        System.out.println(cntNull + " null");
    }

    public void loseType(Object data, Integer i){
        try{
            gainType((Double)data);
        } catch(ClassCastException e){
            cnt++;
            if(e.getMessage() == null){
                cntNull++;
            }
        }
    }

    public void gainType(Double x){

    }
}

Compile with javac Main.java then run with java -Xmixed Main (same as default java Main) and your exception often has a null message. Run it with java -Xint Main and it is never null.

The reason is in mixed mode it uses the runtime interpreter until the class is compiled, then moves over to it, but using -Xint makes it always use the interpreter, in which the message is always present. It seems the compiled native code has a bug that creates the exception without a proper message.

Answer 2

It's not a JVM bug, it's a feature ;) - See NullPointerException in Java with no StackTrace - you need to add the -XX:-OmitStackTraceInFastThrow option to the JVM args and then you'll have stack traces all the time (but slightly slower exception throwing).