Intro

Running external commands in Java is an error prone task. JProc helps managing input and output of external processes as well as error conditions. It uses sensible defaults, such as throwing an exception if a process terminates with a non zero exit code.

Getting Started

To get started either download the jar or if you are using maven add this snippet to your pom:

<dependency>
    <groupId>org.buildobjects</groupId>
    <artifactId>jproc</artifactId>
    <version>2.2.0</version>
</dependency>

For the basic use case of just capturing program output there is a static method:

String output = ProcBuilder.run("echo", "Hello World!");

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

There is another 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", output.trim());

Output and Input

For more control over the execution we'll use a ProcBuilder instance to configure the process.

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!")
    .withOutputStream(output)
    .run();

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")
    .withArgs("-w")
    .withInputStream(input).run();

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!")
    .run();

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());

The Environment

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());

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("/"))
    .run();

assertEquals("/\n", result.getOutputString());

Timeouts

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")
    .withArg("2")
    .withTimeoutMillis(1000);
try {
    builder.run();
    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")
    .withArg("0.5")
    .run();

assertTrue(result.getExecutionTime() > 500 && result.getExecutionTime() < 1000);

In some cases you might want to disable the timeout.

To make this explicit rather than setting the timeout to a very large number there is a method to disable the timeout.

Note: Not having a timeout doesn't necessarily make your system more stable. Especially if the process hangs (e.g. waiting for input on stdin).

ProcBuilder builder = new ProcBuilder("sleep")
    .withArg("7")
    .withNoTimeout();

ProcResult result = builder.run();
assertEquals(result.getExecutionTime(), 7000, 500);

Exit Status

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")
    .withArg("xyz");
try {
    builder.run();
    fail("Should throw exception");
} catch (ExternalProcessFailureException ex) {
    assertEquals("No such file or directory", ex.getStderr().split("\\:")[2].trim());
    assertTrue(ex.getExitValue() > 0);
    assertEquals("ls xyz", ex.getCommand());
    assertTrue(ex.getTime() > 0);

}

In some cases a non-zero exit code doesn't indicate an error, but it is used to return a result, e.g. with grep.

In that case throwing an exception would be inappropriate. To prevent an exception from being thrown we can configure the builder to ignore the exit status:

try {
    ProcResult result = new ProcBuilder("bash")
        .withArgs("-c", "echo Hello World!;exit 100")
        .ignoreExitStatus()
        .run();

    assertEquals("Hello World!\n", result.getOutputString());
    assertEquals(100, result.getExitValue());
} catch (ExternalProcessFailureException ex) {
    fail("A process started with ignoreExitStatus should not throw an exception");
}

It is also possible to specify a set of expected status codes that will not lead to an exception:

try {
    ProcResult result = new ProcBuilder("bash")
        .withArgs("-c", "echo Hello World!;exit 100")
        .withExpectedExitStatuses(0, 100)
        .run();

    assertEquals("Hello World!\n", result.getOutputString());
    assertEquals(100, result.getExitValue());
} catch (ExternalProcessFailureException ex) {
    fail("An expected exit status should not lead to an exception");
}

Satus codes that are not expected will so still lead to an exception:

try {
    ProcResult result = new ProcBuilder("bash")
        .withArgs("-c", "echo Hello World!;exit 99")
        .withExpectedExitStatuses(0, 100)
        .run();

    fail("An exit status that is not part of the expectedExitStatuses should throw");
} catch (ExternalProcessFailureException ex) {
    assertEquals(99, ex.getExitValue());
}

Good to Know

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

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")
    .withInput(data)
    .run();

assertTrue(result.getOutputBytes().length > 2 * MEGA);

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

ProcBuilder builder = new ProcBuilder("date");

String date1 = builder.run().getOutputString();
Thread.sleep(2000);
String date2 = builder.run().getOutputString();

assertNotNull(date1);
assertNotNull(date2);
assertTrue(!date1.equals(date2));

Pipes

Here is how you can consume stdout in a streaming fashion (for example line by line):

new ProcBuilder("echo")
    .withArgs("line1\nline2")
    .withOutputConsumer(new StreamConsumer() {
        public void consume(InputStream stream) throws IOException {
            BufferedReader reader = new BufferedReader(new InputStreamReader(stream));
            assertEquals("line1", reader.readLine());
            assertEquals("line2", reader.readLine());
            assertNull(reader.readLine());
        }
    })
    .withTimeoutMillis(2000)
    .run();

Error output can also be captured

ProcResult result = new ProcBuilder("bash")
    .withArgs("-c", ">&2 echo error;>&2 echo error2; echo out;echo out2")
    .run();

assertEquals("out\nout2\n", result.getOutputString());
assertEquals("error\nerror2\n", result.getErrorString());

Error output can be captured with output stream

ByteArrayOutputStream out = new ByteArrayOutputStream();
ByteArrayOutputStream err = new ByteArrayOutputStream();
new ProcBuilder("bash")
    .withArgs("-c", ">&2 echo error;>&2 echo error2; echo out;echo out2")
    .withOutputStream(out)
    .withErrorStream(err)
    .run();

assertEquals("out\nout2\n", out.toString());
assertEquals("error\nerror2\n", err.toString());