execute java .class file programmatically

[Joel Bijapurkar]

I have written the following code to create the .class file:

import javax.tools.JavaCompiler;
import javax.tools.ToolProvider;


public class CompilerDemo 
{
    public static void main(String args[])
    {
        String fileToCompile = "c:" + java.io.File.separator   + "HelloWorld.java";  
        JavaCompiler compiler =ToolProvider.getSystemJavaCompiler();  
        int compilationResult = compiler.run(null, null, null, fileToCompile);  
        if (compilationResult == 0) {  
            System.out.println("Compilation is successful");  
        } else {  
            System.out.println("Compilation Failed");  
        }  
    }
}

This code runs properly and the HelloWorld.class file does get generated. How do I execute this .class file?

[Henry Wong]

Joel Bijapurkar wrote:I have written the following code to create the .class file:

import javax.tools.JavaCompiler;
import javax.tools.ToolProvider;


public class CompilerDemo 
{
    public static void main(String args[])
    {
        String fileToCompile = "c:" + java.io.File.separator   + "HelloWorld.java";  
        JavaCompiler compiler =ToolProvider.getSystemJavaCompiler();  
        int compilationResult = compiler.run(null, null, null, fileToCompile);  
        if (compilationResult == 0) {  
            System.out.println("Compilation is successful");  
        } else {  
            System.out.println("Compilation Failed");  
        }  
    }
}

This code runs properly and the HelloWorld.class file does get generated. How do I execute this .class file?

Well, since the class file isn't available at compile time (of the program that wants to execute the code), one option is to use the reflection library.

Henry

[Tim Moores]

java.lang.Runtime.exec(...) and java.lang.ProcessBuilder come to mind.

[Joel Bijapurkar]

Thanks Tim and Henry. I tried using the Runtime and ProcessBuilder option. Here is the code i created:

 try
 {
          Runtime re = Runtime.getRuntime();
          theProcess=re.exec(System.getProperty("java.home").toString()+"/bin/java C:/HelloWorld");
 }
 catch(Exception e)
 {
         System.err.println("Error on exec() method");
         e.printStackTrace();
 }

This program executes without any errors or exceptions but the HelloWorld program does not execute. In my HelloWorld program I am writing to a "out.txt" file, but after executing this program when I open the "out.txt" file it is blank.

The HelloWorld program executes fine through the command prompt. But in the command prompt we use the following command set path = "C:\Program File\Java\jdk\bin" . Do I have to do use this command in this case too? If so, how?

[Jeff Verdegan]

Joel Bijapurkar wrote:

          theProcess=re.exec(System.getProperty("java.home").toString()+"/bin/java C:/HelloWorld");

We don't do java file_system_path_to_class_file. We do java -cp some_classpath fully.qualified.ClassName

So, if your HelloWorld class is not in a package, and you have C:/HelloWorld.class, then you'd execute java -cp C:/ HelloWorld

If, on the other hand, HelloWorld was in package com.mycompany.hello, and you had, for example, C:/projects/HW/com/mycompany/hello/HelloWorld.class, then you'd execute java -cp C:/projects/HW com.mycompay.hello.HelloWorld

Note that the class must be in a subdirectory path matching the package name, and the parent directory of the root of that subdirectory must be on the classpath.

[Jeff Verdegan]

Joel Bijapurkar wrote:
The HelloWorld program executes fine through the command prompt.

If you're doing java C:/HelloWorld like you are in your code, then no, it doesn't.

[Joel Bijapurkar]

Thank you Jeff. I tried executing the following command:

theProcess=re.exec("java -cp C:/HelloWorld");

But the file is not executing.

Another problem I am now facing is with the following command:

JavaCompiler compiler =ToolProvider.getSystemJavaCompiler();  

This command works fine in Netbeans but is returning null value when used in the Eclipse IDE.

[Tim Moores]

Try using an absolute path (for "java").

Note that to function properly you may have to handle the input/output/error streams of the Process, and generally follow the advice given in this article.

[Joanne Neal]

Joel Bijapurkar wrote:Thank you Jeff. I tried executing the following command:

theProcess=re.exec("java -cp C:/HelloWorld");

But the file is not executing.

Unfortunately in Jeff's posts a space coinicided with a newline. The command Jeff was suggesting is
java -cp C:/ HelloWorld
with a space after the c:/

[Jeff Verdegan]

Joanne Neal wrote:

Joel Bijapurkar wrote:Thank you Jeff. I tried executing the following command:

theProcess=re.exec("java -cp C:/HelloWorld");

But the file is not executing.

Unfortunately in Jeff's posts a space coinicided with a newline. The command Jeff was suggesting is
java -cp C:/ HelloWorld
with a space after the c:/

Actually, I was making a different point, but that point was based on my own misunderstanding of something the OP had said, so I won't muddy things any further by trying to unravel it.

[Rob Spoor]

Tim Moores wrote:Try using an absolute path (for "java").

Which you can use System.getProperty("java.home") for. Add the bin folder, add the java executable, and you have the full path to the java executable of the currently used JVM.