指定Java代理的类路径

Question

For context, I'm working on a tool called Randoop. What I need to do is to be able to replace certain method calls at runtime using the Java agent. Specifically, I want to replace calls to Random() with Random(0).

I have the following directory structure:

test_randoop/
    randoop.jar
    replacecall.jar
    replacement_file.txt
    ClassWithRandom.java
    ClassWithRandom.class
    replace/
        java/
            util/
                Random.java
                Random.class   // created with "javac Random.java" command

I'm running all commands from the test_randoop directory.

Random.java's contents are:

package replace.java.util;

public class Random {
  /** Default mock for {@code Random()}. Replaces call with {@code Random(0)}. */
  public static java.util.Random randomWithSeedZero() {
    return new java.util.Random(0);
  }

  public static int returnZero() {
    return 0;
  }
}

I want to get the class Random at runtime with the following line where classname is defined as replace.java.util.Random

Class<?> methodClass = Class.forName(classname);

However, this code is being called from the replacecall.jar file which is the Java agent that I'm using to replace calls. This seems to be a problem because the agent doesn't seem to be aware of the replace.java.util.Random class.

To elaborate, the Java program that I'm running is executed with the following command:

java -ea -classpath .:randoop.jar -Xbootclasspath/a:/home/waylonh/test_randoop/replacecall.jar -javaagent:/home/waylonh/test_randoop/replacecall.jar="--replacement-file=replacement_file.txt --debug=true --verbose=true" randoop.main.Main gentests --testclass=ClassWithRandom --output-limit=10

The problem is that the replace.java.util.Random class is never found on the classpath and the forName method throws a ClassNotFoundException .

I've attempted to print out the system classpath at runtime with the following snippet:

ClassLoader classLoader = ClassLoader.getSystemClassLoader();
for (URL u : ((URLClassLoader) classLoader).getURLs()) {
  System.out.println(u.getFile());
}

and the resulting output is:

/home/waylonh/test_randoop/
/home/waylonh/randoop/build/libs/randoop-all-3.1.5.jar
/home/waylonh/randoop/build/libs/replacecall-3.1.5.jar

What could I be missing here? Is there a different classpath that must be provided to the Java agent argument? Why is it that the system class loader lists the test_randoop directory as part of the classpath, and yet the class Random within replace/java/util cannot be found?

Answer 1

Bootstrap classloader (which loads JDK classes) and system classloader (which loads application classes) are different things.

By default, Java agent is loaded by the system classloader, and it has access to the classes specified in -classpath . But in your case the agent is loaded by the bootstrap classloader because of -Xbootclasspath option, and it does not see the classes from the system classloader.

You basically need to remove -Xbootclasspath so that the agent could see user classes, too.