在Android源碼中查找Java代碼中native函數對應的C++實現

Android源碼中很多關鍵代碼都是C++實現的，java通過jni來調用，經常會看到java中這樣的代碼：

    static native Thread currentThread();

如何根據方法名找到其對應的C++實現，有兩個方法。

先來個java代碼的示例VMThread.java：

package java.lang;

class VMThread {
    Thread thread;
    int vmData;

    VMThread(Thread t) {
        thread = t;
    }

    native static void create(Thread t, long stackSize);

    static native Thread currentThread();
    static native boolean interrupted();
    static native void sleep (long msec, int nsec) throws InterruptedException;
    static native void yield();

    native void interrupt();

    native boolean isInterrupted();

    ......
}

 我們要查找currentThread方法的實現。

方法一：

由於Android源碼中對每個native實現都會寫一個java方法名和C++方法名映射的列表，所以我們直接搜索這個列表內容即可。

zkw@zkw $ grep -rns '"currentThread"' ./*
./art/compiler/dex/quick/dex_file_method_inliner.cc:108:    "currentThread",         // kNameCacheCurrentThread
匹配到二進制文件 ./dalvik/vm/native/.java_lang_VMThread.cpp.swp
./dalvik/vm/native/java_lang_VMThread.cpp:241:    { "currentThread",  "()Ljava/lang/Thread;",
./external/android-mock/tests/com/google/android/testing/mocking/AndroidMockGeneratorTest.java:249:    Method method = Thread.class.getMethod("currentThread");
./external/android-mock/tests/com/google/android/testing/mocking/AndroidMockGeneratorTest.java:407:    Method method = Thread.class.getMethod("currentThread");

可以看到，在文件./dalvik/vm/native/java_lang_VMThread.cpp中找到currentThread方法相關的信息，后面()Ljava/lang/Thread代表這個方法的返回值。

進入java_lang_VMThread.cpp這個文件可以看到：

/*
 * java.lang.VMThread
 */
#include "Dalvik.h"
#include "native/InternalNativePriv.h"


/*
 * static void create(Thread t, long stacksize)
 *
 * This is eventually called as a result of Thread.start().
 *
 * Throws an exception on failure.
 */
static void Dalvik_java_lang_VMThread_create(const u4* args, JValue* pResult)
{
    Object* threadObj = (Object*) args[0];
    s8 stackSize = GET_ARG_LONG(args, 1);

    /* copying collector will pin threadObj for us since it was an argument */
    dvmCreateInterpThread(threadObj, (int) stackSize);
    RETURN_VOID();
}

/*
 * static Thread currentThread()
 */
static void Dalvik_java_lang_VMThread_currentThread(const u4* args,
    JValue* pResult)
{
    UNUSED_PARAMETER(args);

    RETURN_PTR(dvmThreadSelf()->threadObj);
}


......

const DalvikNativeMethod dvm_java_lang_VMThread[] = {
    { "create",         "(Ljava/lang/Thread;J)V",
        Dalvik_java_lang_VMThread_create },
    { "currentThread",  "()Ljava/lang/Thread;",
 Dalvik_java_lang_VMThread_currentThread },
    { "getStatus",      "()I",
        Dalvik_java_lang_VMThread_getStatus },
    { "holdsLock",      "(Ljava/lang/Object;)Z",
        Dalvik_java_lang_VMThread_holdsLock },
    { "interrupt",      "()V",
        Dalvik_java_lang_VMThread_interrupt },
    { "interrupted",    "()Z",
        Dalvik_java_lang_VMThread_interrupted },
    { "isInterrupted",  "()Z",
        Dalvik_java_lang_VMThread_isInterrupted },
    { "nameChanged",    "(Ljava/lang/String;)V",
        Dalvik_java_lang_VMThread_nameChanged },
    { "setPriority",    "(I)V",
        Dalvik_java_lang_VMThread_setPriority },
    { "sleep",          "(JI)V",
        Dalvik_java_lang_VMThread_sleep },
    { "yield",          "()V",
        Dalvik_java_lang_VMThread_yield },
    { NULL, NULL, NULL },
};

源碼中第242行找到對應的名字，用紅色標出，其實現就在第44行。

這個方法不是很准確，要靠經驗來判斷搜出來的代碼是否是自己要找的，下一個方法可以較准確的查找。

方法二：

 還是找VMThread.java的currentThread函數，找多了會發現，C++的名字一般都是包名+類名+方法名，比如currentThread的C++名字就肯定包含“java_lang_VMThread_currentThread”，所以直接搜索即可。

grep -rns "java_lang_VMThread_currentThread" ./*