Output is incomplete when running jdb as a process from java application

Question

I'm trying to debug a java class from java program. I created a simple class to test. This is my class hello.java and it lies in the folder C:\Users\madhawax\Desktop\beaufify\debugging

My problem is that I can't retrieve the part

VM Started: Set deferred breakpoint Hello.main
...

when I run jdb from java code, but when I manually run jdb from command line I can see it.

Why do I get only part of the real output? How can I fix this?

This is my Hello.java class:

public class Hello {
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            System.out.println("loop number "+i);
        }
    }
}

I used 3 commands to run jdb

jdb
stop in Hello.main
run Hello

Console output when I debug manually using cmd .

C:\Users\madhawax\Desktop\beaufify\debugging>jdb
Initializing jdb ...
> stop in Hello.main
Deferring breakpoint Hello.main.
It will be set after the class is loaded.
> run Hello
run  Hello
Set uncaught java.lang.Throwable
Set deferred uncaught java.lang.Throwable
>
VM Started: Set deferred breakpoint Hello.main

Breakpoint hit: "thread=main", Hello.main(), line=3 bci=0
3            for (int i = 0; i < 10; i++) {

main[1]

Output when I run jdb using java code .

run:
Initializing jdb ...
> Deferring breakpoint Hello.main.
It will be set after the class is loaded.
> run  Hello
Set uncaught java.lang.Throwable
Set deferred uncaught java.lang.Throwable
> Input stream closed.
BUILD STOPPED (total time: 4 seconds)

I used this code to run jdb.

try {
    ProcessBuilder builder = new ProcessBuilder("C:\\Program Files\\Java\\jdk1.8.0_31\\bin\\jdb.exe");
    builder.directory(new File("C:\\Users\\madhawax\\Desktop\\beaufify\\debugging\\"));
    Process process = builder.start();
    OutputStream stdin = process.getOutputStream();
    InputStream stdout = process.getInputStream();
    BufferedReader reader = new BufferedReader(new InputStreamReader(stdout));
    try (BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(stdin))) {
        writer.write("stop in Hello.main\n");
        writer.flush();
        writer.write("run Hello");
        writer.flush();
    }
    String inputLine;
    Scanner scanner = new Scanner(stdout);
    while (scanner.hasNextLine()) {
        System.out.println(scanner.nextLine());
    }
 } catch (IOException ex) {
    ex.printStackTrace();
}

This doesn't make much sense. Why are you running jdb via Netbeans? Are you aware it has its own debugger? — user207421, Jul 27 '15 at 03:30
@EJP yes yes i know that but that's for my application . i can't use netbean debugger because i should do it programmatically.i'm not going to build a debugger — Madhawa Priyashantha, Jul 27 '15 at 03:33
Your question is not very clear. So, first you run jdb manually from command line, then you run it from your application and you ask why the output is different. Am I right? — default locale, Jul 27 '15 at 03:35
Not sure if it helps, but have you tried `writer.write("run Hello\n");` — default locale, Jul 27 '15 at 03:36
Maybe there's some issue with the end of line encoding? Try using a method like Reader.read() which doesn't block until a new line is detected. — vandale, Jul 27 '15 at 03:41
@FastSnail The "error stream" is conventionally used for anything that's not "normal program output". It's badly named, blame UNIX. — user253751, Jul 27 '15 at 03:53
@immibis i added `error stream` just now.but still same .nooutput came from error stream — Madhawa Priyashantha, Jul 27 '15 at 03:59

default locale · Accepted Answer · 2015-07-27T05:46:14.443

I suggest you to run your code without try-with-resources:

Scanner scanner = new Scanner(stdout);
BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(stdin));
writer.write("stop in Hello.main\n");
writer.flush();
writer.write("run Hello\n");
writer.flush();

while (scanner.hasNextLine()) {
    System.out.println(scanner.nextLine());
}

In your code try-with-resources will close BufferedWriter after execution:

try (BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(stdin)))     {
    writer.write("stop in Hello.main\n");
    writer.flush();
    writer.write("run Hello");
    writer.flush();
} //writer is closed here

Thus, it will close underlying process output stream and this, apparently, leads to closure of jdb process.

You might want to change try-with-resource to try-catch-finally wrapping the whole method.

UPDATE: Also, it's a good idea to read output of one command before running next command. With your approach:

writer.write("stop in Hello.main\n");
writer.flush();
writer.write("run Hello\n");
writer.flush();
writer.write("list\n");
...

There's no pause between command calls. jdb might not be able to handle list command at the time (because it's launching VM). As an experiment you can introduce a time gap:

writer.flush();
Thread.sleep(1000);
writer.write("list\n");

The superior approach is to read output in between.

writer.flush();
readOutput(stdout);
writer.write("list\n");

You can use scanner to read the output. But, as @vandale pointed out in question comments, Scanner blocks on token and line breaks. You might want to use non-blocking reads to read available output. Something like this might work:

private void readOutput(InputStream outputStream) throws IOException{
    byte[] buffer = new byte[100000];
    int bytesRead;
    while (outputStream.available() > 0) {
        bytesRead = outputStream.read(buffer);
        if (bytesRead > 0) {
            System.out.print(new String(buffer, 0, bytesRead));
        }
    }
}

This code will also show output that doesn't end with a line break (input prompts, main[1], etc.)

it worked but i added another command `writer.write("list\n"); writer.flush();` then i'm not getting output for list command but in cmd i got — Madhawa Priyashantha, Jul 27 '15 at 05:21

Output is incomplete when running jdb as a process from java application

1 Answers1