Java Free Reflection

An agent that makes the java reflection API to run perfectly on Java 15+.

When you call setAccessible on some private fields or methods, then JVM throws an exception java.lang.reflect.InaccessibleObjectException: Unable to make... This agent can easily fix it.

Compatibility

• OracleJDK 15~18 works
• OpenJDK 15~18 works

What it did

The agent will hook java.lang.reflect.AccessibleObject.checkCanSetAccessible(Class<?> caller, Class<?> declaringClass, boolean throwExceptionIfDenied) method.

The key implementation is make this method return true, if the reflection caller is our class.

Before hook:

public class AccessibleObject {
    private boolean checkCanSetAccessible(Class<?> caller, Class<?> declaringClass, boolean throwExceptionIfDenied) {
        if (caller == MethodHandle.class) {
            //......
        }
    }
}

After hook:

public class AccessibleObject {
    private boolean checkCanSetAccessible(Class<?> caller, Class<?> declaringClass, boolean throwExceptionIfDenied) {
        if ("org.example.Main".equals(caller.getName())) {
            return true;
        } else if (caller.getName().startsWith("org.xxx.B")) {
            return true;
        } else if (caller.getName().startsWith("org.xxx.C")) {
            return true;
        } else if (caller.getName().startsWith("org.xxx.A")) {
            return true;
        } else if (caller == MethodHandle.class) {
            //......
        }
        //......
    }
}

So it makes reflection can work on inaccessible operations.

How to use

1. Build

   mvn clean package

2. Found your package name or class name which your call reflection api

    package org.example;
    import java.lang.reflect.Field;
    public class Main {
    
        private static void setField(Object object, String fieldName, Object value) throws Exception {
            Class<?> clazz = object.getClass();
            Field field = clazz.getDeclaredField(fieldName);
            field.setAccessible(true);
            field.set(object, value);
        }
        
        public static void main(String[] args) throws Exception {
            // see reflection-exmaple for more details
          }
    
    }

   in above code, package name is org.example, class name is org.example.Main

3. Add argument to your launch options

   Use without free-reflection:

   $ java -jar reflection-example.jar
   ================ Reflection Test ================
   (before) text = Hello world!
   Exception in thread "main" java.lang.reflect.InaccessibleObjectException: Unable to make field private final byte[] java.lang.String.value accessible: module java.base does not "opens java.lang" to unnamed module @a09ee92
   at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:354)
   at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:297)
   at java.base/java.lang.reflect.Field.checkCanSetAccessible(Field.java:178)
   at java.base/java.lang.reflect.Field.setAccessible(Field.java:172)
   at org.example.Main.setField(Main.java:10)
   at org.example.Main.main(Main.java:22)

   Use with free-reflection:(if you want org.exmaple.Main class can free use reflection api):

   $ java -javaagent:free-reflection.jar=log=true;name=org.example.Main -jar reflection-example.jar
   [FreeReflection]  Agent is starting(log=true;name=org.example.Main)...
   [FreeReflection]  Call re-transformClasses(AccessibleObject)
   [FreeReflection]  Transforming...
   [FreeReflection]  Found checkCanSetAccessible
   [FreeReflection]  Injecting...
   [FreeReflection]  Whitelist name : org.example.Main
   [FreeReflection]  Transform successful
   ================ Reflection Test ================
   (before) text = Hello world!
   (after) text = China NO.1.
   ================ Reflection Test ================

More Examples

• disable log output

  $ java -javaagent:free-reflection.jar=log=true -jar reflection-example.jar

• use for multi class names

  $ java -javaagent:free-reflection.jar=log=true;name=org.xxx.ClassA,org.xxx.ClassB,org.xxx.ClassC -jar 
  reflection-example.jar

• use for multi package prefix

  $ java -javaagent:free-reflection.jar=log=true;prefix=org.pkg.aaa,org.pkg.bbb,org.pkg.ccc -jar 
  reflection-example.jar

• full usage

  $ java -javaagent:free-reflection.jar=log=true;name=org.xxx.ClassA,org.xxx.ClassB,org.xxx.ClassC;prefix=org.pkg.aaa,org.pkg.bbb,org.pkg.ccc -jar 
  reflection-example.jar

License

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.