问题描述

Android 的 JNI 提示页面提到了这个 FAQ:为什么 FindClass 没有找到我的课程?他们提到了多种解决方案，最后一个选项是这个:

Android's JNI tips page mentions this FAQ: Why didn't FindClass find my class? They mention multiple solutions and the last option there is this one:

在方便的地方缓存对 ClassLoader 对象的引用，然后发出loadClass 直接调用.这需要一些努力.

Cache a reference to the ClassLoader object somewhere handy, and issue loadClass calls directly. This requires some effort.

所以，我试图让它工作，但似乎无论如何，这种方法对我来说根本不起作用.最终，我想出了如何使用 ClassLoader 但如果我从本机线程尝试加载尚未触摸/加载的类，它将无法工作.本质上，它在从本机线程调用时的行为与 env->FindClass 相同，但它不会为已在应用程序中使用的类返回 0.任何想法，如果我没有做对，或者不可能从尚未使用/加载的本机线程访问类.

So, I tried to get it working and it seems that no matter what, this method simply does not work for me. Eventually, I figured how to use ClassLoader but it won't work if from a native thread I try to loadClass that hasn't been touched/loaded yet. Essentially, it's the identical to env->FindClass in behavior when called from a native thread, with the exception that it won't return 0 for classes that were already use in the app. Any idea if I didn't get it right, or it's impossible to access classes from a native thread that weren't used/loaded yet.

我将提供更多信息来解释我的意思.有常规的 JNI env->FindClass(className)，还有一个我写的 myFindClass(env, className) 使用缓存的 ClassLoader->loadClass.

I'll give more info to explain what exactly I mean. There is regular JNI env->FindClass(className), and another one that I wrote myFindClass(env, className) that uses cached ClassLoader->loadClass.

我试图从本机 c/c++ 访问的类是com/noname/TestClient".在 myFindClass 中，我还使用 env->FindClass 并记录它返回的值:

The class that I'm trying to access from native c/c++ is "com/noname/TestClient". Inside myFindClass I also use env->FindClass and log value that it returns:

jclass myFindClass(JNIEnv * env, const char* name)
{
    ...
    jclass c0 = env->FindClass(name);
    jclass c1 = (jclass)env->CallObjectMethod(ClassLoader,
        MID_loadClass, envNewStringUTF(name));
    dlog("myFindClass("%s") => c0:%p, c1:%p, c0 and c1 are same: %d",
        name, c0, c1, env->IsSameObject(c0, c1));
    ...
}

那么，我有这 3 个组合来解释这个问题.

Then, I have these 3 combinations to explain the issue.

1)

//inside JNI_OnLoad thread
myFindClass(env, "com/noname/TestClient");
...

//inside native thread created by pthread_create
myFindClass(env, "com/noname/TestClient");

我得到了这个 logcat:

I get this logcat:

myFindClass("com/noname/TestClent") => c0:0x41b64558, c1:0x41b64558,c0 和 c1 相同:1
...
myFindClass("com/noname/TestClent") => c0:0,c1:0x41b64558，c0和c1相同:0

myFindClass("com/noname/TestClent") => c0:0x41b64558, c1:0x41b64558, c0 and c1 are same: 1
...
myFindClass("com/noname/TestClent") => c0:0, c1:0x41b64558, c0 and c1 are same: 0

2)

//inside JNI_OnLoad thread
env->FindClass("com/noname/TestClient");
...

//inside native thread created by pthread_create
myFindClass("com/noname/TestClient");

我得到了这个 logcat:

I get this logcat:

myFindClass("com/noname/TestClent") => c0:0, c1:0x41b64558, c0 和 c1 相同:0

myFindClass("com/noname/TestClent") => c0:0, c1:0x41b64558, c0 and c1 are same: 0

3)

//inside JNI_OnLoad thread
//"com/noname/TestClient" isn't touched from JNI_OnLoad.
...

//inside native thread created by pthread_create
myFindClass(env, "com/noname/TestClient");

我得到了这个 logcat:

I get this logcat:

myFindClass("com/noname/TestClent") => c0:0, c1:0, c0 和 c1 相同:1

myFindClass("com/noname/TestClent") => c0:0, c1:0, c0 and c1 are same: 1

基本上，我的问题是 ClassLoader 在第三种情况下找不到我的课程.它是一个错误吗?有什么办法可以解决这个问题?

Basically, my issue is that ClassLoader doesn't find my class in the 3rd case. Is it a bug? What can be done to fix the problem?

最重要的是，似乎 ClassLoader::loadClass 显然是错误的.如果我问 myFindClass("noname/TestClent") 那么它会返回一些垃圾，当我以任何方式使用返回的 jclass 时，应用程序就会崩溃.

On top of that, it seems that ClassLoader::loadClass is plainly buggy. If I ask myFindClass("noname/TestClent") then it returns some garbage, and when I use that returned jclass in any way the app crashes.

推荐答案

经过多次尝试和我的应用程序崩溃后，我和一位同事设法在另一个本地线程中缓存并成功使用类加载器.我们使用的代码如下所示(C++11，但很容易转换为 C++2003)，发布在这里，因为我们找不到上述在方便的地方缓存对 ClassLoader 对象的引用，并发出 loadClass直接调用.这需要一些努力.".当从与 JNI_OnLoad 线程不同的线程调用时，调用 findClass 可以完美运行.我希望这会有所帮助.

After much trying and crashing of my app, a colleague and I managed to cache and succesfully use the class loader in another, native, thread. The code we used is shown below (C++11, but easily converted to C++2003), posted here since we couldn't find any examples of the aforementioned "Cache a reference to the ClassLoader object somewhere handy, and issue loadClass calls directly. This requires some effort.". Calling findClass worked perfectly when called from a thread different from the one of JNI_OnLoad. I hope this helps.

JavaVM* gJvm = nullptr;
static jobject gClassLoader;
static jmethodID gFindClassMethod;

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *pjvm, void *reserved) {
    gJvm = pjvm;  // cache the JavaVM pointer
    auto env = getEnv();
    //replace with one of your classes in the line below
    auto randomClass = env->FindClass("com/example/RandomClass");
    jclass classClass = env->GetObjectClass(randomClass);
    auto classLoaderClass = env->FindClass("java/lang/ClassLoader");
    auto getClassLoaderMethod = env->GetMethodID(classClass, "getClassLoader",
                                             "()Ljava/lang/ClassLoader;");
    gClassLoader = env->CallObjectMethod(randomClass, getClassLoaderMethod);
    gFindClassMethod = env->GetMethodID(classLoaderClass, "findClass",
                                    "(Ljava/lang/String;)Ljava/lang/Class;");

    return JNI_VERSION_1_6;
}

jclass findClass(const char* name) {
    return static_cast<jclass>(getEnv()->CallObjectMethod(gClassLoader, gFindClassMethod, getEnv()->NewStringUTF(name)));
}

JNIEnv* getEnv() {
    JNIEnv *env;
    int status = gJvm->GetEnv((void**)&env, JNI_VERSION_1_6);
    if(status < 0) {    
        status = gJvm->AttachCurrentThread(&env, NULL);
        if(status < 0) {        
            return nullptr;
        }
    }
    return env;
}

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