Java Runtime.getRuntime(): getting output from executing a command line program

Process management & Wrangling outputs

Working with external processes can feel like dealing with aliens. You don't know their language, you don't know their technology, but you've got to somehow communicate. Here's how you do it without causing an intergalactic incident:

Command and control with ProcessBuilder

This ain't your grandpa's Runtime.exec(). ProcessBuilder lets you play boss and exercise more control over the process:

ProcessBuilder builder = new ProcessBuilder("your-command");
builder.redirectErrorStream(true); // Because nobody likes to face errors alone
Process process = builder.start();
// Rest of your code...
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Streamlined Output Management

Don't let your streams cross! To avoid a buffer overflow ghost, use the snapshot rule - read the output and the error simultaneously:

// We came, we saw, we conquered - the stdout
new Thread(() -> {
    // BufferedReader code to consume stdout
}).start();

// Who you gonna call? Error busters!
new Thread(() -> {
    // BufferedReader code to handle stderr
}).start();


Converting & Parsing Output

To easy your journey in the process wilderness, there's the ever-reliable IOUtils from Apache commons-io to convert the output stream to a String:

String output = IOUtils.toString(process.getInputStream(), StandardCharsets.UTF_8); // Voila! Output in hand
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Manual Execution & Permissions

Sure, it's a manual car, but do you know how to drive it? Test your command outside of Java to ensure it runs succesfully and your Java application has allowed permissions.

Stream Management & Character Encoding

Like a river, streams flow effortlessly... if you manage them right! Remember - stream management is not a stream of consciousness. It requires planning. Here's how:

Encoding: Lost in Translation?

Just as you won't understand French if you don't know the language, use InputStreamReader with the correct character encoding to prevent your output from becoming gibberish:

BufferedReader reader = new BufferedReader(
  new InputStreamReader(process.getInputStream(), StandardCharsets.UTF_8) // Learning the process language
);
// Rest of your code...
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Stream Buffers: The Leaky Faucet Problem

Streams can overflow easily. If you don't want to call plumber Java to your rescue, watch the buffer size carefully:

// Your code to handle large output streams ...
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Resource Management: Cleaning After Yourself

Nobody likes a memory hog. With try-with-resources, Java 7+ does the clean-up duty for you:

try (
  BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream())) // Let's open that stream
) {
    // Your code goes here...
} // And... stream's closed!
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Handling Errors: Oopsie-daisies!

No one likes errors! But they do happen. Keeping tabs on Process.getErrorStream() helps you handle the unexpected:

BufferedReader reader = new BufferedReader(
  new InputStreamReader(process.getErrorStream()) // Prep the emergency error response team
);
// Rest of your code...
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Navigating the edge cases

In the hinterland of processes, you may trip over the unexpected. Let's identify these booby traps:

The Stream Deadlocks

For streaming outputs longer than a Tolstoy novel, consider asynchronous read or PipedInputStream/PipedOutputStream. Safety first!

Handling Ugly Processes

What's more restless than a toddler hopped up on sugar? A long-running process! Handle them with grace:

if (!process.waitFor(1, TimeUnit.MINUTES)) { // If the process overstays its welcome
    // Timeout logic
}
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Robust Error Handling & Testing

Your code should run like a seasoned globe trotter - well in all environments and with all command-line tools.