Java Top

I just announced the new Learn Spring course, focused on the fundamentals of Spring 5 and Spring Boot 2:

>> CHECK OUT THE COURSE

1. Overview

The Process API provides a powerful way to execute operating system commands in Java. However, it has several options that can make it cumbersome to work with.

In this tutorial, we’ll take a look at how Java alleviates that with the ProcessBuilder API.

2. ProcessBuilder API

The ProcessBuilder class provides methods for creating and configuring operating system processes. Each ProcessBuilder instance allows us to manage a collection of process attributes. We can then start a new Process with those given attributes.

Here are a few common scenarios where we could use this API:

  • Find the current Java version
  • Set-up a custom key-value map for our environment
  • Change the working directory of where our shell command is running
  • Redirect input and output streams to custom replacements
  • Inherit both of the streams of the current JVM process
  • Execute a shell command from Java code

We’ll take a look at practical examples for each of these in later sections.

But before we dive into the working code, let’s take a look at what kind of functionality this API provides.

2.1. Method Summary

In this section, we’re going to take a step back and briefly look at the most important methods in the ProcessBuilder class. This will help us when we dive into some real examples later on:

  • ProcessBuilder(String... command)

    To create a new process builder with the specified operating system program and arguments, we can use this convenient constructor.

  • directory(File directory)

    We can override the default working directory of the current process by calling the directory method and passing a File object. By default, the current working directory is set to the value returned by the user.dir system property.

  • environment()

    If we want to get the current environment variables, we can simply call the environment method. It returns us a copy of the current process environment using System.getenv() but as a Map.

  • inheritIO()

    If we want to specify that the source and destination for our subprocess standard I/O should be the same as that of the current Java process, we can use the inheritIO method.

  • redirectInput(File file), redirectOutput(File file), redirectError(File file)

    When we want to redirect the process builder’s standard input, output and error destination to a file, we have these three similar redirect methods at our disposal.

  • start()

    Last but not least, to start a new process with what we’ve configured, we simply call start().

We should note that this class is NOT synchronized. For example, if we have multiple threads accessing a ProcessBuilder instance concurrently then the synchronization must be managed externally.

3. Examples

Now that we have a basic understanding of the ProcessBuilder API, let’s step through some examples.

3.1. Using ProcessBuilder to Print the Version of Java

In this first example, we’ll run the java command with one argument in order to get the version.

Process process = new ProcessBuilder("java", "-version").start();

First, we create our ProcessBuilder object passing the command and argument values to the constructor. Next, we start the process using the start() method to get a Process object.

Now let’s see how to handle the output:

List<String> results = readOutput(process.getInputStream());

assertThat("Results should not be empty", results, is(not(empty())));
assertThat("Results should contain java version: ", results, hasItem(containsString("java version")));

int exitCode = process.waitFor();
assertEquals("No errors should be detected", 0, exitCode);

Here we’re reading the process output and verifying the contents is that we expect. In the final step, we wait for the process to finish using process.waitFor().

Once the process has finished, the return value tells us whether the process was successful or not.

A few important points to keep in mind:

  • The arguments must be in the right order
  • Moreover, in this example, the default working directory and environment are used
  • We deliberately don’t call process.waitFor() until after we’ve read the output because the output buffer might stall the process
  • We’ve made the assumption that the java command is available via the PATH variable

3.2. Starting a Process With a Modified Environment

In this next example, we’re going to see how to modify the working environment.

But before we do that let’s begin by taking a look at the kind of information we can find in the default environment:

ProcessBuilder processBuilder = new ProcessBuilder();        
Map<String, String> environment = processBuilder.environment();
environment.forEach((key, value) -> System.out.println(key + value));

This simply prints out each of the variable entries which are provided by default:

PATH/usr/bin:/bin:/usr/sbin:/sbin
SHELL/bin/bash
...

Now we’re going to add a new environment variable to our ProcessBuilder object and run a command to output its value:

environment.put("GREETING", "Hola Mundo");

processBuilder.command("/bin/bash", "-c", "echo $GREETING");
Process process = processBuilder.start();

Let’s decompose the steps to understand what we’ve done:

  • Add a variable called ‘GREETING’ with a value of ‘Hola Mundo’ to our environment which is a standard Map<String, String>
  • This time, rather than using the constructor we set the command and arguments via the command(String… command) method directly.
  • We then start our process as per the previous example.

To complete the example, we verify the output contains our greeting:

List<String> results = readOutput(process.getInputStream());
assertThat("Results should not be empty", results, is(not(empty())));
assertThat("Results should contain java version: ", results, hasItem(containsString("Hola Mundo")));

3.3. Starting a Process With a Modified Working Directory

Sometimes it can be useful to change the working directory. In our next example we’re going to see how to do just that:

@Test
public void givenProcessBuilder_whenModifyWorkingDir_thenSuccess() 
  throws IOException, InterruptedException {
    ProcessBuilder processBuilder = new ProcessBuilder("/bin/sh", "-c", "ls");

    processBuilder.directory(new File("src"));
    Process process = processBuilder.start();

    List<String> results = readOutput(process.getInputStream());
    assertThat("Results should not be empty", results, is(not(empty())));
    assertThat("Results should contain directory listing: ", results, contains("main", "test"));

    int exitCode = process.waitFor();
    assertEquals("No errors should be detected", 0, exitCode);
}

In the above example, we set the working directory to the project’s src dir using the convenience method directory(File directory). We then run a simple directory listing command and check that the output contains the subdirectories main and test.

3.4. Redirecting Standard Input and Output

In the real world, we will probably want to capture the results of our running processes inside a log file for further analysis. Luckily the ProcessBuilder API has built-in support for exactly this as we will see in this example.

By default, our process reads input from a pipe. We can access this pipe via the output stream returned by Process.getOutputStream().

However, as we’ll see shortly, the standard output may be redirected to another source such as a file using the method redirectOutput. In this case, getOutputStream() will return a ProcessBuilder.NullOutputStream.

Let’s return to our original example to print out the version of Java. But this time let’s redirect the output to a log file instead of the standard output pipe:

ProcessBuilder processBuilder = new ProcessBuilder("java", "-version");

processBuilder.redirectErrorStream(true);
File log = folder.newFile("java-version.log");
processBuilder.redirectOutput(log);

Process process = processBuilder.start();

In the above example, we create a new temporary file called log and tell our ProcessBuilder to redirect output to this file destination.

In this last snippet, we simply check that getInputStream() is indeed null and that the contents of our file are as expected:

assertEquals("If redirected, should be -1 ", -1, process.getInputStream().read());
List<String> lines = Files.lines(log.toPath()).collect(Collectors.toList());
assertThat("Results should contain java version: ", lines, hasItem(containsString("java version")));

Now let’s take a look at a slight variation on this example. For example when we wish to append to a log file rather than create a new one each time:

File log = tempFolder.newFile("java-version-append.log");
processBuilder.redirectErrorStream(true);
processBuilder.redirectOutput(Redirect.appendTo(log));

It’s also important to mention the call to redirectErrorStream(true). In case of any errors, the error output will be merged into the normal process output file.

We can, of course, specify individual files for the standard output and the standard error output:

File outputLog = tempFolder.newFile("standard-output.log");
File errorLog = tempFolder.newFile("error.log");

processBuilder.redirectOutput(Redirect.appendTo(outputLog));
processBuilder.redirectError(Redirect.appendTo(errorLog));

3.5. Inheriting the I/O of the Current Process

In this penultimate example, we’ll see the inheritIO() method in action. We can use this method when we want to redirect the sub-process I/O to the standard I/O of the current process:

@Test
public void givenProcessBuilder_whenInheritIO_thenSuccess() throws IOException, InterruptedException {
    ProcessBuilder processBuilder = new ProcessBuilder("/bin/sh", "-c", "echo hello");

    processBuilder.inheritIO();
    Process process = processBuilder.start();

    int exitCode = process.waitFor();
    assertEquals("No errors should be detected", 0, exitCode);
}

In the above example, by using the inheritIO() method we see the output of a simple command in the console in our IDE.

In the next section, we’re going to take a look at what additions were made to the ProcessBuilder API in Java 9.

4. Java 9 Additions

Java 9 introduced the concept of pipelines to the ProcessBuilder API:

public static List<Process> startPipeline​(List<ProcessBuilder> builders)

Using the startPipeline method we can pass a list of ProcessBuilder objects. This static method will then start a Process for each ProcessBuilder. Thus, creating a pipeline of processes which are linked by their standard output and standard input streams.

For example, if we want to run something like this:

find . -name *.java -type f | wc -l

What we’d do is create a process builder for each isolated command and compose them into a pipeline:

@Test
public void givenProcessBuilder_whenStartingPipeline_thenSuccess()
  throws IOException, InterruptedException {
    List builders = Arrays.asList(
      new ProcessBuilder("find", "src", "-name", "*.java", "-type", "f"), 
      new ProcessBuilder("wc", "-l"));

    List processes = ProcessBuilder.startPipeline(builders);
    Process last = processes.get(processes.size() - 1);

    List output = readOutput(last.getInputStream());
    assertThat("Results should not be empty", output, is(not(empty())));
}

In this example, we’re searching for all the java files inside the src directory and piping the results into another process to count them.

To learn about other improvements made to the Process API in Java 9, check out our great article on Java 9 Process API Improvements.

5. Conclusion

To summarize, in this tutorial, we’ve explored the java.lang.ProcessBuilder API in detail.

First, we started by explaining what can be done with the API and summarized the most important methods.

Next, we took a look at a number of practical examples. Finally, we looked at what new additions were introduced to the API in Java 9.

As always, the full source code of the article is available over on GitHub.

Java bottom

I just announced the new Learn Spring course, focused on the fundamentals of Spring 5 and Spring Boot 2:

>> CHECK OUT THE COURSE