Java Lib to Launch External Processes

I recently redesigned some of the code I tend to use to spawn external processes (pdflatex anyone?) in java. The implementation is still a bit buggy, but I am more interested in people’s opinions about the API (non-blocking killable invocations are not yet supported). The project on github is called jproc. Here is the cookbook so far:

Get the library from maven central:


To launch an external program we’ll use a ProcBuilder. The run method
builds and spawns the actual process and blocks until the process exits.
The process takes care of writing the output to a stream (as opposed to the standard
facilities in the JDK that expect the client to actively consume the
output from an input stream:

ByteArrayOutputStream output = new ByteArrayOutputStream();
new ProcBuilder("echo")
        .withArg("Hello World!")
assertEquals("Hello World!\n", output.toString());

The input can be read from an arbitrary input stream, like this:

ByteArrayInputStream input = new ByteArrayInputStream("Hello cruel World".getBytes());
ProcResult result = new ProcBuilder("wc")
assertEquals("3", result.getOutputString().trim());

If all you want to get is the string that gets returned and if there
is not a lot of data, using a streams is quite cumbersome. So for convenience
if no stream is provdied the output is captured by default and can be
obtained from the result.

ProcResult result = new ProcBuilder("echo")
                            .withArg("Hello World!")
assertEquals("Hello World!\n", result.getOutputString());
assertEquals(0, result.getExitValue());
assertEquals("echo \"Hello World!\"", result.getProcString());

For providing input there is a convenience method too:

ProcResult result = new ProcBuilder("cat")
   .withInput("This is a string").run();
assertEquals("This is a string", result.getOutputString());

Some external programs are using environment variables. These can also
be set using the withVar method

ProcResult result = new ProcBuilder("bash")
                            .withArgs("-c", "echo $MYVAR")
                            .withVar("MYVAR","my value").run();
assertEquals("my value\n", result.getOutputString());
assertEquals("bash -c \"echo $MYVAR\"", result.getProcString());

A common usecase for external programs is batch processing of data.
These programs might always run into difficulties. Therefore a timeout can be
specified. There is a default timeout of 5000ms. If the program does not terminate
within the timeout interval it will be terminated and the failure is indicated through
an exception:

ProcBuilder builder = new ProcBuilder("sleep")
try {;
    fail("Should time out");
catch (TimeoutException ex){
    assertEquals("Process 'sleep 2' timed out after 1000ms.", ex.getMessage());

Even if the process does not timeout, we might be interested in the
execution time. It is also available through the result:

ProcResult result = new ProcBuilder("sleep")
assertTrue(result.getExecutionTime() > 500 && result.getExecutionTime() < 1000);

By default the new program is spawned in the working directory of
the parent process. This can be overidden:

ProcResult result = new ProcBuilder("pwd")
        .withWorkingDirectory(new File("/"))
assertEquals("/\n", result.getOutputString());

It is a time honoured tradition that programs signal a failure
by returning a non-zero exit value. However in java failure is
signalled through exceptions. Non-Zero exit values therefore
get translated into an exception, that also grants access to
the output on standard error.

ProcBuilder builder = new ProcBuilder("ls")
try {;
    fail("Should throw exception");
} catch (ExternalProcessFailureException ex){
    assertEquals("ls: xyz: No such file or directory\n", ex.getStderr());
    assertEquals(1, ex.getExitValue());
    assertEquals("ls xyz", ex.getCommand());
    assertEquals("ls: xyz: No such file or directory\n", ex.getStderr());
    assertTrue(ex.getTime() > 0);

Input and output can also be provided as byte[].
ProcBuilder also copes with large amounts of

int MEGA = 1024 * 1024;
byte[] data = new byte[4 * MEGA];
for (int i = 0; i < data.length; i++) {
    data[i] = (byte) Math.round(Math.random() * 255 - 128);
ProcResult result = new ProcBuilder("gzip")
assertTrue(result.getOutputBytes().length > 2 * MEGA);

The builder allows to build and spawn several processes from
the same builder instance:

ProcBuilder builder = new ProcBuilder("uuidgen");
String uuid1 =;
String uuid2 =;

For convenience there is also a static method that just runs a
program and captures the ouput:

String output ="echo", "Hello World!");
assertEquals("Hello World!\n", output);

Also there is a static method that filters a given string through
a program:

String output = ProcBuilder.filter("x y z","sed" ,"s/y/a/");
assertEquals("x a z\n", output);






4 responses to “Java Lib to Launch External Processes”

  1. Tiest Vilee Avatar
    Tiest Vilee

    I love it. pity I’m using c# now.

  2. wirzi Avatar

    pretty cool, nice interface, i’ll definitely consider using the lib,
    i.e. when substituting xml-fo with latex

    .. did u notice:

  3. Jens Schauder Avatar

    This sucks … I wrote tons of ugly code a couple of months ago, which could have been that much nicer with this API. Like it.

  4. szczepan Avatar

    awesome library!!! Thanks for doing this. Something like that should be a part of JDK IMHO.

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