usernugget/class-redefiner

Library to modify classes at runtime

class-redefiner

Library to modify classes at runtime using Instrumentation

Limitations

• Require Java 11+ to work
• CDS will ignore modified classes (see this and this)

Getting started

1. Add dependency

   • Using Gradle

     implementation("io.github.usernugget:class-redefiner:2.1.1")

   • Using Maven

     <dependency>
       <groupId>io.github.usernugget</groupId>
       <artifactId>class-redefiner</artifactId>
       <version>2.1.1</version>
     </dependency>

Examples

1. Basic:

// Mapping that will modify ClassValue behavior
@Mapping(targetClass = ClassValue.class)
public static final class ClassValueMapping {
  // Inject code on top of ClassValue::alwaysTrue
  @Head
  public static void alwaysTrue() {
    Op.returnOp(false);
  }
}

public static final class ClassValue {
  // Basic method that always return true
  public static boolean alwaysTrue() {
    return true;
  }
}

// Create new redefiner
ClassRedefiner redefiner = new ClassRedefiner(
  new StandardAttachTypes(),
  new StandardHandlerTypes()
);

// Initialize javaagent
try {
  redefiner.initializeAgent();
} catch (Throwable throwable) {
  throw new IllegalStateException("unable to initialize javaagent", throwable);
}

// Redefine ClassValue using ClassValueMapping
try {
  redefiner.transformClass(ClassValueMapping.class);
} catch (Throwable throwable) {
  throw new IllegalStateException("unable to apply mapping", throwable);
}

// Verify that ClassValue::alwaysTrue now return false
System.out.println("alwaysTrue is " + ClassValue.alwaysTrue());