Intro

Running external commands in Java is an error prone task. JProc helps managing input and output of non-interactive, 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 status.

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>2.8.2</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.getCommandLine());

If you want to set multiple environment variables, you can pass a Map:

Map<String, String> envVariables = new HashMap<>();
envVariables.put("var1", "val 1");
envVariables.put("var2", "val 2");
ProcResult result = new ProcBuilder("bash")
        .withArgs("-c", "env")
        .withVars(envVariables).run();

assertTrue(result.getOutputString().contains("var1=val 1\n"));
assertTrue(result.getOutputString().contains("var2=val 2\n"));
assertEquals("bash -c env", result.getCommandLine());

The environment can be cleared of values inherited from the parent process:

Map<String, String> envVariables = new HashMap<>();
envVariables.put("var1", "val 1");
envVariables.put("var2", "val 2");
ProcResult result = new ProcBuilder("bash")
        .withArgs("-c", "env")
        .clearEnvironment()
        .withVars(envVariables).run();

String[] outputLines = result.getOutputString().split("\n");
assertEquals("var1=val 1", outputLines[0]);
assertEquals("var2=val 2", outputLines[1]);
// Note: environment is not going to be completely empty, as there are some variables that every process needs
//       thus we only assert on the first two lines.

assertEquals("bash -c env", result.getCommandLine());

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.getCommandLine());
    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");
}

Status 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();

Of course, you can consume stderr in the same way:

new ProcBuilder("bash")
    .withArgs("-c", ">&2 echo error;>&2 echo error2;echo stdout")
    .withOutputConsumer(new StreamConsumer() {
        @Override
        public void consume(InputStream stream) throws IOException {
            BufferedReader reader = new BufferedReader(new InputStreamReader(stream));
            assertEquals("stdout", reader.readLine());
            assertNull(reader.readLine());
        }
    })
    .withErrorConsumer(new StreamConsumer() {
        @Override
        public void consume(InputStream stream) throws IOException {
            BufferedReader reader = new BufferedReader(new InputStreamReader(stream));
            assertEquals("error", reader.readLine());
            assertEquals("error2", reader.readLine());
            assertNull(reader.readLine());
        }
    })
    .withTimeoutMillis(2000)
    .run();

Error output can also be accessed directly:

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());

Alteratively an output stream can be passed in:

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());

String Representations

The builder can also return a string representation of the invocation. Naturally this method doesn’t support chaining, that means you’ll have to store the builder in a variable to finally run the process.

final ProcBuilder echoBuilder = new ProcBuilder("echo")
    .withArgs("Hello World!");

assertEquals("echo 'Hello World!'", echoBuilder.getProcString());

ProcResult result = echoBuilder.run();
assertEquals("Hello World!\n", result.getOutputString());