how to get the output of a java program?

Question

I need to write a code that could help me to get the output of a program.

I have in that Java file MainClass.java that I am compiling, a simple code that prints "OK" .

How could I return this output and even if there is error, I need to take it as well.

This is my code that compile and create a .class file.

File sourceFile = new File("C:\\Projet_Interne\\webProject\\NewFolder\\MainClass.java");  
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
int compilationResult = compiler.run(null, null, null,sourceFile.getPath());
int result = compiler.run(System.in,System.out,System.err,sourceFile.getPath());
     System.out.println("Compile result code = " + result);
      if(compilationResult == 0){
            System.out.println("Compilation is successful");
      }else{
             System.out.println("Compilation Failed");
      }
 StandardJavaFileManager stdFileManager = compiler.getStandardFileManager(null, Locale.getDefault(), null);
 stdFileManager.setLocation(StandardLocation.CLASS_OUTPUT, Arrays.asList(new File("C:\\Projet_Interne\\webProject\\NewFolder")));

Answer 1

The run method can use streams to handle input and output:

int run(InputStream in,
      OutputStream out,
      OutputStream err,
      String... arguments)

Use an OutputStream for out and read from it when run is finished.

Answer 2

You could use the DiagnosticCollector , which will allow you to collect diagnostic information about the compilation process, you can find more details from the JavaDocs

For example...

File helloWorldJava = new File(...);

DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<JavaFileObject>();
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager fileManager = compiler.getStandardFileManager(diagnostics, null, null);

// This sets up the class path that the compiler will use.
// I've added the .jar file that contains the DoStuff interface within in it...
List<String> optionList = new ArrayList<String>();
optionList.add("-classpath");
optionList.add(System.getProperty("java.class.path"));

Iterable<? extends JavaFileObject> compilationUnit
        = fileManager.getJavaFileObjectsFromFiles(Arrays.asList(helloWorldJava));
JavaCompiler.CompilationTask task = compiler.getTask(
    null, 
    fileManager, 
    diagnostics, 
    optionList, 
    null, 
    compilationUnit);
if (task.call()) {
    System.out.println("Yipe");
} else {
    // Opps compile failed...
    for (Diagnostic<? extends JavaFileObject> diagnostic : diagnostics.getDiagnostics()) {
        System.out.format("Error on line %d in %s%n",
                diagnostic.getLineNumber(),
                diagnostic.getSource().toUri());
    }
}
fileManager.close();

Once compiled, you have two choices, you can either use a custom class loader to load the class and execute it, which will use your current stdout...

// Create a new custom class loader, pointing to the directory that contains the compiled
// classes, this should point to the top of the package structure!
URLClassLoader classLoader = new URLClassLoader(new URL[]{new File("./").toURI().toURL()});
// Load the class from the classloader by name....
Class<?> loadedClass = classLoader.loadClass("testcompile.HelloWorld");
// Create a new instance...
Object obj = loadedClass.newInstance();
// Santity check
if (obj instanceof DoStuff) {
    // Cast to the DoStuff interface
    DoStuff stuffToDo = (DoStuff)obj;
    // Run it baby
    stuffToDo.doStuff();
}

Or use a ProcessBuilder to execute another Java process...

ProcessBuilder pb = new ProcessBuilder("java", "HelloWorld");
pb.directory(new File("src"));
pb.redirectError();
Process p = pb.start();

InputStreamConsumer.consume(p.getInputStream());

p.waitFor();

For reference, the InputStreamConsumer ....

public static class InputStreamConsumer implements Runnable {

    private InputStream is;

    public InputStreamConsumer(InputStream is) {
        this.is = is;
    }

    public InputStream getInputStream() {
        return is;
    }

    public static void consume(InputStream is) {
        InputStreamConsumer consumer = new InputStreamConsumer(is);
        Thread t = new Thread(consumer);
        t.start();
    }

    @Override
    public void run() {
        InputStream is = getInputStream();
        int in = -1;
        try {
            while ((in = is.read()) != -1) {
                System.out.print((char)in);
            }
        } catch (IOException exp) {
            exp.printStackTrace();
        }
    }

}

This is outlined in more detail in:

• How do you dynamically compile and load external java classes?
• How to compile and run inside the java program