Java process problem in Eclipse

Question

Hi!

This is my code:

final String run_tool ="cmd.exe /C pelda.exe";
final Process tool_proc = null;

Runnable doRun = new Runnable() {
    public void run() {
        try {
            tool_proc = Runtime.getRuntime().exec(run_tool);
            } 
        catch (IOException e) {
            e.printStackTrace();
        }
    }
};    

Thread th = new Thread(doRun);
th.start();

InputStream toolstr = tool_proc.getInputStream();

After the eclipse gives this warning message fot the tool_proc variable:

The final local variable tool_proc cannot be assigned, since it is defined in an enclosing type

I dont't know why my code doesn't work

please help me

Answer 1

Initial value of final field cannot be changed. Remove final modificator from tool_proc:

Process tool_proc = null;

Answer 2

The tool_proc variable is final - final variables can only be assigned once, and you already assigned it the value null in the declaration. Removing the null assignment won't work because the runnable can be called more than once, resulting in multiple assignment. So your best option is to remove the final keyword and it should compile, at least:

Answer 3

The final modifier forbids changing the variable once it has been set. Kevin gives an excellent explanation of why you can't use it in this context.

The construction you have chosen for requires you to introduce a field in the parent class and set it via run():

class MyClass {
    Process tool_proc = null;

    void myFunction() {
        final String run_tool ="cmd.exe /C pelda.exe";
        Runnable doRun = new Runnable() {
            public void run() {
                try {
                    tool_proc = Runtime.getRuntime().exec(run_tool);
                    } 
                catch (IOException e) {
                    e.printStackTrace();
                }
            }
        };
        Thread th = new Thread(doRun);
        th.start();

        // tool_proc will be null at this point!
        InputStream toolstr = tool_proc.getInputStream();
    }
}

In the above code I have demonstrated that tool_proc will always be null on the last line, as the thread has in all liklihood not finished its execution by this time!

You can expect NullPointerExceptions!

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It seems what you are trying to do is very similar to this article about capturing application output asynchronously. It's a good read, and explains why conventional approaches can cause problems. I have an implementation very similar to their StreamGobbler (article page 4).