Name: dk106046 Date: 08/13/2003

java version "1.4.2-beta"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2-beta-b06)
Java HotSpot(TM) Client VM (build 1.4.2-beta-b06, mixed mode)

Compile and execute this multi threaded testcase.

Ex: java Test 5

You can see the testcase hanging. Press Ctrl+\ to get the Javacore and see a dead lock detected between Object TestObj and java.io.PrintStream.

Test.java


public class Test extends Thread
{
    private static TestObj testObj = new TestObj("This is a test.");
    private static int loopNum = 100;

    public void run()
    {
        for(int i=0; i<loopNum; i++)
        {
            testObj.test();
            //passing an object to System.out.println might cause deadlock
            //if the object has a synchronized toString() method.
            //using System.out.println(testObj.toString()) won't have a problem.

           System.out.println(testObj);

        }
    }

    public static void main(String args[])
    {
        if (args.length>2 || args.length<1)
        {
            System.out.println("Usage: Test thread_number [loop_number]");
            System.exit(1);
        }

        int num = new Integer(args[0]).intValue();
        if(args.length>1)
            loopNum = new Integer(args[1]).intValue();

        Test[] t = new Test[num];
        for(int i=0; i<num; i++)
        {
            t[i] = new Test();
            t[i].start();
        }

        try
        {

            for(int i=0; i <num; i++)
                t[i].join();
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
        System.out.println("Test completed");
    }
}

class TestObj
{
    String mStr;

    TestObj(String str)
    {
        mStr = str;
    }

    synchronized void test()
    {
        try
        {
            long t = Math.round(Math.random()*10);
            Thread.currentThread().sleep(t);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }

        //the following line might cause hang if there is System.out.println(testObj)
        //called by other threads.
        System.out.println("In test().");
    }

    synchronized public String toString()
    {
           System.out.println("Calling toString\n");
        return mStr;
    }

}

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