CODEWITHSUNDEEP
javajvm
Madhav Kumar
Madhav Kumar

Reputation: 21

Running a .class file from java code

How can I run the compiled code (.class) java from the java code itself?

I'm doing a kind of providing service like compiling and running java code on server side and giving output to the end user.

Can anyone suggest an approach that will accomplish this?

import java.io.*;

public class demo {

    public static void main(String args[]) throws IOException, InterruptedException {
        int result;
        try {

            System.out.println("command output:");
            Process proc = Runtime.getRuntime().exec("java -cp . demoh");
            InputStream in = proc.getInputStream();
            result = proc.waitFor();

            BufferedInputStream buffer = new BufferedInputStream(proc.getInputStream());

            BufferedReader commandOutput = new BufferedReader(new InputStreamReader(buffer));
            System.out.print(commandOutput);

            String line = null;
            try {
                while ((line = commandOutput.readLine()) != null) {
                    System.out.print(line);
                    System.out.println("command output: " + line);
            }//end while
            commandOutput.close();

            } catch (IOException e) {
                //log and/or handle it  
            }
        } catch (IOException e) {
            System.err.println("IOException raised: " + e.getMessage());
        }
    }
}

Upvotes: 1

Views: 5006

Answers (6)

Tudor
Tudor

Reputation: 62439

If you have the .class files somewhere on disk, simply spawn a new process and run the java command like you would from a command line:

Process p = Runtime.getRuntime().exec("java <java class file>");

After some testing around, the following code worked for me:

public static void main(String args[]) throws IOException, InterruptedException {
    int result;

    try {

        System.out.println("command output:");
        Process proc = Runtime.getRuntime().exec("java -cp . Test");

        InputStream errin = proc.getErrorStream();
        InputStream in = proc.getInputStream(); 
        BufferedReader errorOutput = new BufferedReader(new InputStreamReader(errin));
        BufferedReader output = new BufferedReader(new InputStreamReader(in));
        String line1 = null;
        String line2 = null;
        try {
            while ((line1 = errorOutput.readLine()) != null || 
                   (line2 = output.readLine()) != null) {                   
                if(line1 != null) System.out.print(line1);
                if(line2 != null) System.out.print(line2);               
            }//end while
            errorOutput.close();
            output.close();
        } catch (IOException e) {
            e.printStackTrace(); 
        }//end catc
        result = proc.waitFor();
    } catch (IOException e) {
        System.err.println("IOException raised: " + e.getMessage());
    }
}

Note two things here:

  1. The runtime errors given by the java process are sent to the error stream, not input stream, so you have to read both of them!
  2. You have to read the stream as the process is running. Waiting for the process to finish before reading the streams causes a deadlock because the process output buffer is filled and is waiting for your parent process to read the data, while the parent is waiting for the child to finish!

Upvotes: 1

Robert J Saulnier
Robert J Saulnier

Reputation: 51

Here's a sample app that will compile the Java source file, load the class, instantiate an instance, and print out the toString() of the class HelloWorld. I believe you'll need tools.jar on the classpath. The sample code expects the source file in the src folder. The src folder is required on the classpath since the .class file will get generated there by default.

For more control of the Java Compiler, read up on the javax.tools package.

package sourcerunner;

import java.io.IOException;
import java.util.concurrent.TimeUnit;
import javax.tools.JavaCompiler;
import javax.tools.ToolProvider;

public class SourceRunner {

    public static void main(String[] args) throws IOException, ClassNotFoundException, InstantiationException, IllegalAccessException, InterruptedException {
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();

        compiler.run(System.in, System.out, System.err, "src/sourcerunner/HelloWorld.java");

        TimeUnit.SECONDS.sleep(1L);

        Class<?> cls = Class.forName("sourcerunner.HelloWorld");

        Object instance = cls.newInstance();

        System.out.println(instance);
    }
}

And here's the HelloWorld class:

package sourcerunner;

public class HelloWorld {

    @Override
    public String toString() {
        return "Hello Java Compiler World.";
    }
}

The above code is insanely insecure. Once you understand the code, modify it to use a new ClassLoader to load and instantiate the class. Make sure the ClassLoader has minimal permissions.

Upvotes: 0

OleksandrC
OleksandrC

Reputation: 1

One of the options is to create an instance of the class using the class loader. The class loader can take your class as a byte array and then you can create an instance of it and run it. See this method in the JDK docs.

Upvotes: 0

siamii
siamii

Reputation: 24084

Try 

Process process = Runtime.getRuntime().exec("java " + filePath); // without .class
Scanner output = new Scanner(process.getInputStream());
while (output.hasNext) {
   String token = output.next();
   ...
}

Upvotes: 0

Stefan
Stefan

Reputation: 12453

There are still a lot of building/compiling tools, i.e. Ant or Maven, you can check before you write your own.

Upvotes: 0

DGomez
DGomez

Reputation: 1480

Create .jar file and add that file to the build path

Upvotes: 0

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