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在Android源码中查找Java代码中native函数对应的C++实现
Android源码中很多关键代码都是C++实现的，java通过jni来调用，经常会看到java中这样的代码：
static native Thread currentThread();
如何根据方法名找到其对应的C++实现，有两个方法。
先来个java代码的示例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();
......}
我们要查找currentThread方法的实现。
方法一：
由于Android源码中对每个native实现都会写一个java方法名和C++方法名映射的列表，所以我们直接搜索这个列表内容即可。
zkw@zkw $ grep -rns ‘"currentThread"‘ ./*
./art/compiler/dex/quick/dex_file_method_inliner.cc:108: "currentThread", // kNameCacheCurrentThread
匹配到二进制文件 ./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方法相关的信息，后面()Ljava/lang/Thread代表这个方法的返回值。
进入java_lang_VMThread.cpp这个文件可以看到：
17 /* 18 * java.lang.VMThread 19 */ 20 #include "Dalvik.h" 21 #include "native/InternalNativePriv.h" 22 23 24 /* 25 * static void create(Thread t, long stacksize) 26 * 27 * This is eventually called as a result of Thread.start(). 28 * 29 * Throws an exception on failure. 30 */ 31 static void Dalvik_java_lang_VMThread_create(const u4* args, JValue* pResult) 32 { 33 Object* threadObj = (Object*) args[0]; 34 s8 stackSize = GET_ARG_LONG(args, 1); 35 36 /* copying collector will pin threadObj for us since it was an argument */ 37 dvmCreateInterpThread(threadObj, (int) stackSize); 38 RETURN_VOID(); 39 } 40 41 /* 42 * static Thread currentThread() 43 */ 44 static void Dalvik_java_lang_VMThread_currentThread(const u4* args, 45 JValue* pResult) 46 { 47 UNUSED_PARAMETER(args); 48 49 RETURN_PTR(dvmThreadSelf()->threadObj); 50 } 51
......
237 238 const DalvikNativeMethod dvm_java_lang_VMThread[] = {239 { "create", "(Ljava/lang/Thread;J)V",240 Dalvik_java_lang_VMThread_create },241 { "currentThread", "()Ljava/lang/Thread;",242 Dalvik_java_lang_VMThread_currentThread },243 { "getStatus", "()I",244 Dalvik_java_lang_VMThread_getStatus },245 { "holdsLock", "(Ljava/lang/Object;)Z",246 Dalvik_java_lang_VMThread_holdsLock },247 { "interrupt", "()V",248 Dalvik_java_lang_VMThread_interrupt },249 { "interrupted", "()Z",250 Dalvik_java_lang_VMThread_interrupted },251 { "isInterrupted", "()Z",252 Dalvik_java_lang_VMThread_isInterrupted },253 { "nameChanged", "(Ljava/lang/String;)V",254 Dalvik_java_lang_VMThread_nameChanged },255 { "setPriority", "(I)V",256 Dalvik_java_lang_VMThread_setPriority },257 { "sleep", "(JI)V",258 Dalvik_java_lang_VMThread_sleep },259 { "yield", "()V",260 Dalvik_java_lang_VMThread_yield },261 { NULL, NULL, NULL },262 };
源码中第242行找到对应的名字，用红色标出，其实现就在第44行。
这个方法不是很准确，要靠经验来判断搜出来的代码是否是自己要找的，下一个方法可以较准确的查找。
方法二：
还是找VMThread.java的currentThread函数，找多了会发现，C++的名字一般都是包名+类名+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" ./*
在Android源码中查找Java代码中native函数对应的C++实现