ProcessBuilder vs. Runtime.exec(): When to Use Which for Executing External Commands?

Understanding the Difference between ProcessBuilder and Runtime.exec()

When executing external commands from Java code, developers often encounter two common methods: Runtime.getRuntime().exec(...) and new ProcessBuilder(...).start(). While these methods seem similar, they have key differences that can impact the execution of commands.

Overloading and Tokenization

Runtime.exec() offers both single-string and array overloads. When using the single-string overload, the supplied string is tokenized into an array of arguments. This tokenization behavior doesn't apply to ProcessBuilder. ProcessBuilder constructors only accept varargs arrays or lists of strings, assuming each string represents an individual argument.

Impact on Command Execution

Let's illustrate this difference with an example. On Windows, the following Runtime.exec() invocation:

Runtime.getRuntime().exec("C:\DoStuff.exe -arg1 -arg2");

will execute the "DoStuff.exe" program with the arguments "-arg1" and "-arg2". Tokenization ensures that the command is properly parsed.

In contrast, the following ProcessBuilder invocation will fail unless a program named "DoStuff.exe -arg1 -arg2" exists in the C: directory:

ProcessBuilder b = new ProcessBuilder("C:\DoStuff.exe -arg1 -arg2");

To execute the command correctly using ProcessBuilder, you must either provide the arguments separately:

ProcessBuilder b = new ProcessBuilder("C:\DoStuff.exe", "-arg1", "-arg2");

Or use a list:

List<String> params = java.util.Arrays.asList("C:\DoStuff.exe", "-arg1", "-arg2");
ProcessBuilder b = new ProcessBuilder(params);

Implications for Error Handling

The differences in command tokenization can affect error handling. For example, if ProcessBuilder fails to find the specified program, you may get an error code of 1001 instead of the expected exit value of 0. Understanding the tokenization behavior of Runtime.exec() and ProcessBuilder is crucial for debugging such errors.

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