Adding jar file to instrumentation path

Question

I have two jar files (for the example lets call them Updater.jar and Code.jar). Updater.jar is launched with its main method, and then it launches itself again with a premain method:

package Update;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.ArrayList;
import java.util.List;

public class InstructionLauncher {

    private List<UpdateInstruction> instructions = new ArrayList<UpdateInstruction>();
    private static InstructionLauncher instance;
    private Process process;

    public static InstructionLauncher initialise(){
        if(instance !=null) return instance;
        else return new InstructionLauncher();
    }

    public void registerPremain(UpdateInstruction inst){
        instructions.add(inst);
    }

    public void launchNext(){
        UpdateInstruction inst = instructions.get(0);
        String cls = inst.getClassName() + "." + inst.getMethodName();
        String[] args = new String[]{"java", "-javaagent", "JOSUpdater.jar", "-jar", inst.getClassName() + "." + inst.getMethodName()};
        ProcessBuilder builder = new ProcessBuilder(args);
        try {
            exportResource(cls, cls);
        } catch (Exception e) {
            UpdateManager.revert();
        }
        try {
            Process p = builder.start();
            process = p;
        } catch (IOException e) {
            e.printStackTrace();
        }
        while(!process.isAlive())launchNext();
    }

    private InstructionLauncher(){
        instance = this;
    }

    //From http://stackoverflow.com/questions/10308221/how-to-copy-file-inside-jar-to-outside-the-jar
    private String exportResource(String resourceName, String clazz) throws Exception {
        InputStream stream = null;
        OutputStream resStreamOut = null;
        String jarFolder;
        try {
            stream = Class.forName(clazz).getResourceAsStream(resourceName);//note that each / is a directory down in the "jar tree" been the jar the root of the tree
            if(stream == null) {
                throw new Exception("Cannot get resource \"" + resourceName + "\" from Jar file.");
            }

            int readBytes;
            byte[] buffer = new byte[4096];
            jarFolder = new File(Class.forName(clazz).getProtectionDomain().getCodeSource().getLocation().toURI().getPath()).getParentFile().getPath().replace('\\', '/');
            resStreamOut = new FileOutputStream(jarFolder + resourceName);
            while ((readBytes = stream.read(buffer)) > 0) {
                resStreamOut.write(buffer, 0, readBytes);
            }
        } catch (Exception ex) {
            throw ex;
        } finally {
            stream.close();
            resStreamOut.close();
        }

        return jarFolder + resourceName;
    }

}

The premain method looks like this at the moment:

package Update;

import java.lang.instrument.Instrumentation;

public class PremainLauncher {

    public static void premain(String args, Instrumentation inst){
        inst.addTransformer(new Transformer(), true);
        System.out.println("Registered instruction for package: " + args);
    }

}

What I am wondering, is how do I add the whole external JAR (Code.jar in this example) into the path for the instrumentation?

I know about the Instrumentation.retransformClasses method, but to use that I would need to get a List> of all the classes in the jar, which I have been unable to complete.

Lets say that Code.jar has three class files: Main.class, writer.class and display.class. Is there a way to get a list of each of their class object, not their name?

Answer 1

A Java agent can add jar files simply via the Instrumentation interface it received in the startup method, e.g.

import java.io.IOException;
import java.lang.instrument.Instrumentation;
import java.util.jar.JarFile;

public class PremainLauncher {
    public static void premain(String args, Instrumentation inst) throws IOException{
        inst.appendToSystemClassLoaderSearch(new JarFile("Code.jar"));
        inst.addTransformer(new Transformer(), true);
        System.out.println("Registered instruction for package: " + args);
    }
}

See Instrumentation.appendToSystemClassLoaderSearch(…). If you intent to instrument JRE classes as well in a way that the instrumented classes need access to the classes of Code.jar, you’ll have to change the bootstrap path instead.

Note that this has to happen as early as possible:

The Java™ Virtual Machine Specification specifies that a subsequent attempt to resolve a symbolic reference that the Java virtual machine has previously unsuccessfully attempted to resolve always fails with the same error that was thrown as a result of the initial resolution attempt. Consequently, if the JAR file contains an entry that corresponds to a class for which the Java virtual machine has unsuccessfully attempted to resolve a reference, then subsequent attempts to resolve that reference will fail with the same error as the initial attempt.