How to get an object instance of a class

OSC invites you to party! 1028 Suzhou Yuan Founding Association, let’s hunt for treasures in the AI era

How to get an object instance of a Java class? This class is not necessarily a singleton, nor does it necessarily provide static methods, nor is it necessarily managed by Spring. How can we obtain all object instances of this class when its source code cannot even be modified? Here is an implementation method based on JVMTI.

Instructions for use

First quote maven dependencies

<dependency>
   <groupId>io.github.liubsyy</groupId>
  <artifactId>FindInstancesOfClass</artifactId>
   <version>1.0.1</version>
</dependency>

Then directly call the function InstancesOfClass.getInstances(Class<?> targetClass) to get all object instances of a class

public class InstancesOfClass {
    /**
     * native方法 : 返回所有的实例对象
     * @param targetClass 需要查询实例的Class
     * @return
     */
    public static native Object[] getInstances(Class<?> targetClass);
}

Implementation principle

There is no interface for obtaining instances based on classes in Java. You need to use the JVMTI interfaces IterateOverInstancesOfClass and GetObjectsWithTags.

First write a class containing native methods

public class InstancesOfClass {
    /**
     * native方法 : 返回所有的实例对象
     * @param targetClass 需要查询实例的Class
     * @return
     */
    public static native Object[] getInstances(Class<?> targetClass);
}

Then use javah to generate the .h file, and then use C++ to write the implementation part.

#include <jni.h>
#include <jvmti.h>
#include "com_liubs_findinstances_jvmti_InstancesOfClass.h"


static jvmtiIterationControl JNICALL objectInstanceCallback(jlong class_tag, jlong size, jlong* tag_ptr, void* user_data) {
    *tag_ptr = 1;
    return JVMTI_ITERATION_CONTINUE;
}

JNIEXPORT jobjectArray JNICALL Java_com_liubs_findinstances_jvmti_InstancesOfClass_getInstances(JNIEnv* env, jclass clazz, jclass targetClazz) {
    JavaVM* vm;
    env->GetJavaVM(&vm);

    jvmtiEnv* jvmti;
    vm->GetEnv((void**)&jvmti, JVMTI_VERSION_1_0);

    jvmtiCapabilities capabilities = {0};
    capabilities.can_tag_objects = 1;
    jvmti->AddCapabilities(&capabilities);

    jvmti->IterateOverInstancesOfClass(targetClazz, JVMTI_HEAP_OBJECT_EITHER,
                                       objectInstanceCallback, NULL);

    jlong tag = 1;
    jint count;
    jobject* instances;
    jvmti->GetObjectsWithTags(1, &tag, &count, &instances, NULL);

    printf("Found %d objects with tag\n", count);

    // 转换jobject* 为 jobjectArray 并返回
    jobjectArray result = env->NewObjectArray(count, targetClazz, NULL);
    for (int i = 0; i < count; i++) {
        env->SetObjectArrayElement(result, i, instances[i]);
    }

    jvmti->Deallocate((unsigned char*)instances);
    return result;
}

Then use gcc/g++ to compile the cpp source code to generate the corresponding dynamic link library files under linux/mac/windows. so, .dylib and .dll, load the corresponding local link library through System.load(), and finally call InstancesOfClass.getInstances(Class <?> targetClass) method.

For detailed source code, please see https://github.com/Liubsyy/FindInstancesOfClass , which contains test cases.