JNI源码分析 （并实现JNI动态注册）

原文出自：http://blog.csdn.net/urrjdg/article/details/78091094

1 . C/C++ 的 编译 和 链接

c/c++ ========= 二进制文件
对于C/C++ 一般分为两个阶段
1. 编译
xxx.c ——> windows .obj ； Linux .o –》 语法检查
链接
.o —–> log.so .dll .exe
举例：
a.c a.h b.c b.h
a.c –>b.h(test方法)
在编译阶段只会去找b.h有没有test方法，
而在链接的阶段，他会在b.o当中去找这个test方法
如果没有test方法会 报 LinkErro错误。
而这个Link erro 错误一般是因为，我们在一个文件当中引入了一个.h文件，并且使用了这个文件当中的这个方法，而这个对应的.h文件对应的.o文件（中间文件）里面没有这个方法的实现体。

2.编译器

将这个C/C++编译链接生成二进制文件的这个过程是谁做的？

是编译器

编译规则：

Eclipse

GUN编译器 —-> 编译规则 Android.mk （log.so是android自带的）

Android Studio

LLVM编译器 —-> 编译规则 CMakeList.txt

三段式编译器

3 . 使用android studio 创建一个工程

勾上 android studio 会给我们提供一个 exceptiosns support 异常支持

public class MainActivity extends AppCompatActivity {

private static String TAG = "MainActivity";

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

// Example of a call to a native method
TextView tv = (TextView) findViewById(R.id.sample_text);
//        tv.setText(stringFromJNI());
diff();
}

public void diff(){
Log.d(TAG,"diff ");
FileUtils.diff("a","b",2);
}

}

javah 生成头文件

public class FileUtils {

public static native void diff(String path,String pattern_Path,int file_num);

public static void javaDiff(String path,String pattern_Path,int file_num){}

// Used to load the 'native-lib' library on application startup.
static {
System.loadLibrary("native-lib");
}

}

#include “com_example_zeking_lsn9_FileUtils.h”

#include <android/log.h>

//int __android_log_print(int prio, const char *tag,  const char *fmt, ...)
#define TAG "Zeking_JNI"
// __VA_ARGS__  代表可以输入参数  %s  %d 之类的
#define LOGI(...) __android_log_print(ANDROID_LOG_INFO,TAG,__VA_ARGS__)

/*
* Class:     com_example_zeking_lsn9_FileUtils
* Method:    diff
* Signature: (Ljava/lang/String;Ljava/lang/String;I)V
*/
JNIEXPORT void JNICALL Java_com_example_zeking_lsn9_FileUtils_diff
(JNIEnv *env, jclass clazz, jstring path, jstring pattern_Path, jint file_num){

LOGI("JNI Begin....%s..","Zeking  Hello");
}

由上产生的疑问：

jvm是虚拟机内存

C/C++是native内存

并且这个so库是放在apk的lib下面的

那这个so库 ，系统是怎么找到的

System.loadLibrary是怎么来找到的？

并且系统是如何来区分（JVM是怎么来区分native 方法（diff）和 javaDiff方法）

native关键字起到什么作用？

loadLibrary做了什么？

当我们调用javaDiff的时候会到Java虚拟机的内存当中来处理找这个方法，而加了native关键字的时候他就会去到C++的堆栈空间找这个C++的实现。

为什么native会这样，起了什么作用？

先在看声明了native的方法和没有声明native方法之间的区别。

使用 javap -s -p -v FileUtils.class。

找到这两个方法，可以看到这两个方法的区别在于 flag ，native声明的方法 多了个 ACC_NATIVE 的flag。

也就是说java在执行这个文件的时候，对于有ACC_NATIVE的flag的方法，他就会去native区间去找，如果没有ACC_NATIVE 这个flag 就在本地的虚拟机空间来找这个方法

————可以通过使用javap -s -p -v FileUtils.class命令查看flag标志位————————

C:\Users\Zeking\Desktop\Lsn9\app\src\main\java\com\example\zeking\lsn9>javap -s -p -v FileUtils.class

Classfile /C:/Users/Zeking/Desktop/Lsn9/app/src/main/java/com/example/zeking/lsn9/FileUtils.class
Last modified 2017-9-2; size 469 bytes
MD5 checksum 19201ed5479758e0dfffb63528653a65
Compiled from "FileUtils.java"
public class com.example.zeking.lsn9.FileUtils
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref          #5.#16         // java/lang/Object."<init>":()V
#2 = String             #17            // native-lib
#3 = Methodref          #18.#19        // java/lang/System.loadLibrary:(Ljava/lang/String;)V
#4 = Class              #20
4000
// com/example/zeking/lsn9/FileUtils
#5 = Class              #21            // java/lang/Object
#6 = Utf8               <init>
#7 = Utf8               ()V
#8 = Utf8               Code
#9 = Utf8               LineNumberTable
#10 = Utf8               diff
#11 = Utf8               (Ljava/lang/String;Ljava/lang/String;I)V
#12 = Utf8               javaDiff
#13 = Utf8               <clinit>
#14 = Utf8               SourceFile
#15 = Utf8               FileUtils.java
#16 = NameAndType        #6:#7          // "<init>":()V
#17 = Utf8               native-lib
#18 = Class              #22            // java/lang/System
#19 = NameAndType        #23:#24        // loadLibrary:(Ljava/lang/String;)V
#20 = Utf8               com/example/zeking/lsn9/FileUtils
#21 = Utf8               java/lang/Object
#22 = Utf8               java/lang/System
#23 = Utf8               loadLibrary
#24 = Utf8               (Ljava/lang/String;)V
{
public com.example.zeking.lsn9.FileUtils();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1                  // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 7: 0

public static native void diff(java.lang.String, java.lang.String, int);
descriptor: (Ljava/lang/String;Ljava/lang/String;I)V
flags: ACC_PUBLIC, ACC_STATIC, ACC_NATIVE  // 这边多了个 ACC_NATIVE 代表是native

public static void javaDiff(java.lang.String, java.lang.String, int);
descriptor: (Ljava/lang/String;Ljava/lang/String;I)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=0, locals=3, args_size=3
0: return
LineNumberTable:
line 11: 0

static {};
descriptor: ()V
flags: ACC_STATIC
Code:
stack=1, locals=0, args_size=0
0: ldc           #2                  // String native-lib
2: invokestatic  #3                  // Method java/lang/System.loadLibrary:(Ljava/lang/String;)V
5: return
LineNumberTable:
line 15: 0
line 16: 5
}

SourceFile: “FileUtils.java”

4 . System.loadLibrary 找到so库文件 分析

native的方法栈为什么能被jvm调用到？从System.loadLibrary 入手

System.loadLibrary("native-lib");

System.java
public static void loadLibrary(String libname) {
Runtime.getRuntime().loadLibrary0(VMStack.getCallingClassLoader(), libname);
}

Runtime.java

synchronized void loadLibrary0(ClassLoader loader, String libname) {
if (libname.indexOf((int)File.separatorChar) != -1) {
throw new UnsatisfiedLinkError(
"Directory separator should not appear in library name: " + libname);
}
String libraryName = libname;
if (loader != null) {
// 点进去发现是return null；找到so库的全路径
String filename = loader.findLibrary(libraryName);
if (filename == null) {
// It's not necessarily true that the ClassLoader used
// System.mapLibraryName, but the default setup does, and it's
// misleading to say we didn't find "libMyLibrary.so" when we
// actually searched for "liblibMyLibrary.so.so".
throw new UnsatisfiedLinkError(loader + " couldn't find \"" +
System.mapLibraryName(libraryName) + "\"");
}
String error = doLoad(filename, loader);
if (error != null) {
throw new UnsatisfiedLinkError(error);
}
return;
}

String filename = System.mapLibraryName(libraryName);
List<String> candidates = new ArrayList<String>();
String lastError = null;
for (String directory : getLibPaths()) {
String candidate = directory + filename;
candidates.add(candidate);

if (IoUtils.canOpenReadOnly(candidate)) {
String error = doLoad(candidate, loader);
if (error == null) {
return; // We successfully loaded the library. Job done.
}
lastError = error;
}
}

if (lastError != null) {
throw new UnsatisfiedLinkError(lastError);
}
throw new UnsatisfiedLinkError("Library " + libraryName + " not found; tried " + candidates);
}

String filename = loader.findLibrary(libraryName);

点进去 发现是 return null；

ClassLoader.java

protected String findLibrary(String libname) {
return null;
}

所以可以想到 应该是 ClassLoader的实现类去实现了这个 findLibrary方法。

怎么找是哪个实现类 实现的呢？

Log.i(TAG,this.getClassLoader().toString());

dalvik.system.PathClassLoader[DexPathList[[zip file "/data/app/com.example.zeking.lsn9-1/base.apk", zip file "/data/app/com.example.zeking.lsn9-1/split_lib_dependencies_apk.apk", zip file "/data/app/com.example.zeking.lsn9-1/split_lib_slice_0_apk.apk", zip file "/data/app/com.example.zeking.lsn9-1/split_lib_slice_1_apk.apk", zip file "/data/app/com.example.zeking.lsn9-1/split_lib_slice_2_apk.apk", zip file "/data/app/com.example.zeking.lsn9-1/split_lib_slice_3_apk.apk", zip file "/data/app/com.example.zeking.lsn9-1/split_lib_slice_4_apk.apk", zip file "/data/app/com.example.zeking.lsn9-1/split_lib_slice_5_apk.apk", zip file "/data/app/com.example.zeking.lsn9-1/split_lib_slice_6_apk.apk", zip file "/data/app/com.example.zeking.lsn9-1/split_lib_slice_7_apk.apk", zip file "/data/app/com.example.zeking.lsn9-1/split_lib_slice_8_apk.apk", zip file "/data/app/com.example.zeking.lsn9-1/split_lib_slice_9_apk.apk"],nativeLibraryDirectories=[/data/app/com.example.zeking.lsn9-1/lib/arm64, /data/app/com.example.zeking.lsn9-1/base.apk!/lib/arm64-v8a, /data/app/com.example.zeking.lsn9-1/split_lib_dependencies_apk.apk!/lib/arm64-v8a, /data/app/com.example.zeking.lsn9-1/split_lib_slice_0_apk.apk!/lib/arm64-v8a, /data/app/com.example.zeking.lsn9-1/split_lib_slice_1_apk.apk!/lib/arm64-v8a, /data/app/com.example.zeking.lsn9-1/split_lib_slice_2_apk.apk!/lib/arm64-v8a, /data/app/com.example.zeking.lsn9-1/split_lib_slice_3_apk.apk!/lib/arm64-v8a, /data/app/com.example.zeking.lsn9-1/split_lib_slice_4_apk.apk!/lib/arm64-v8a, /data/app/com.example.zeking.lsn9-1/split_lib_slice_5_apk.apk!/lib/arm64-v8a, /data/app/com.example.zeking.lsn9-1/split_lib_slice_6_apk.apk!/lib/arm64-v8a, /data/app/com.example.zeking.lsn9-1/split_lib_slice_7_apk.apk!/lib/arm64-v8a, /data/app/com.example<
18154
/span>.zeking.lsn9-1/split_lib_slice_8_apk.apk!/lib/arm64-v8a, /data/app/com.example.zeking.lsn9-1/split_lib_slice_9_apk.apk!/lib/arm64-v8a, /vendor/lib64, /system/lib64]]]

从上面可以看出是 PathClassLoader

PathClassLoader .java 这里面没有 findLibrary 继续进到 BaseDexClassLoader

public class PathClassLoader extends BaseDexClassLoader {
......
}

BaseDexClassLoader .java

private final DexPathList pathList;
/**
* Constructs an instance.
*
* @param dexPath the list of jar/apk files containing classes and
* resources, delimited by {@code File.pathSeparator}, which
* defaults to {@code ":"} on Android
* @param optimizedDirectory directory where optimized dex files
* should be written; may be {@code null}
* @param librarySearchPath the list of directories containing native
* libraries, delimited by {@code File.pathSeparator}; may be
* {@code null}
* @param parent the parent class loader
*/
public BaseDexClassLoader(String dexPath, File optimizedDirectory,
String librarySearchPath, ClassLoader parent) {
super(parent);
this.pathList = new DexPathList(this, dexPath, librarySearchPath, optimizedDirectory);
}

@Override
public String findLibrary(String name) {
return pathList.findLibrary(name);
}

DexPathList .java

public String findLibrary(String libraryName) {
String fileName = System.mapLibraryName(libraryName);

for (Element element : nativeLibraryPathElements) {
String path = element.findNativeLibrary(fileName);

if (path != null) {
return path;
}
}

return null;
}

首先我们先来看

DexPathList .java 中的 String fileName = System.mapLibraryName(libraryName);

System.java 看注释可以看出 ，是 根据你的平台来找你的 so库

/**
* Maps a library name into a platform-specific string representing
* a native library.
*
* @param      libname the name of the library.
* @return     a platform-dependent native library name.
* @exception  NullPointerException if <code>libname</code> is
*             <code>null</code>
* @see        java.lang.System#loadLibrary(java.lang.String)
* @see        java.lang.ClassLoader#findLibrary(java.lang.String)
* @since      1.2
*/
public static native String mapLibraryName(String libname);

再继续看

for (Element element : nativeLibraryPathElements) {

从DexPathList .java 可以看到 nativeLibraryPathElements 是在 DexPathList的构造函数里面初始化的

public DexPathList(ClassLoader definingContext, String dexPath,
String librarySearchPath, File optimizedDirectory) {

......
// 找so库是从两个地方来找，
// 1.在BaseDexClassLoader初始化的时候传入的目录 这个目录是 librarySearchPath，这个就是应用apk下面的解压的lib目录下
// 2. 在系统的环境变量里面，System.getProperty("java.library.path")：这个目录通过Log.i(TAG,System.getProperty("java.library.path"));打印 出来是 /vendor/lib64:/system/lib64 或者  /vendor/lib:/system/lib

// dalvik.system.PathClassLoader[DexPathList[[zip file "/data/app/com.example.zeking.lsn9-1.apk"],nativeLibraryDirectories=[/data/app-lib/com.example.zeking.lsn9-1, /system/lib]]]
// /data/app-lib/com.example.zeking.lsn9-1,
// /system/lib

this.nativeLibraryDirectories = splitPaths(librarySearchPath, false);
// 这个是系统里面 java.library.path
this.systemNativeLibraryDirectories =
splitPaths(System.getProperty("java.library.path"), true);
List<File> allNativeLibraryDirectories = new ArrayList<>(nativeLibraryDirectories);
allNativeLibraryDirectories.addAll(systemNativeLibraryDirectories);
// 就是在这边进行初始化的
this.nativeLibraryPathElements = makePathElements(allNativeLibraryDirectories,
suppressedExceptions,
definingContext);

......
}

5 . System.loadLibrary

加载so库文件 分析

分析下他是怎么加载so库的

现在回到Runtime.java 的 loadLibrary0 方法 找到他的doLoad 方法

synchronized void loadLibrary0(ClassLoader loader, String libname) {
if (libname.indexOf((int)File.separatorChar) != -1) {
throw new UnsatisfiedLinkError(
"Directory separator should not appear in library name: " + libname);
}
String libraryName = libname;
if (loader != null) {
String filename = loader.findLibrary(libraryName); // 找到so库的全路径
if (filename == null) {
// It's not necessarily true that the ClassLoader used
// System.mapLibraryName, but the default setup does, and it's
// misleading to say we didn't find "libMyLibrary.so" when we
// actually searched for "liblibMyLibrary.so.so".
throw new UnsatisfiedLinkError(loader + " couldn't find \"" +
System.mapLibraryName(libraryName) + "\"");
}
String error = doLoad(filename, loader);
if (error != null) {
throw new UnsatisfiedLinkError(error);
}
return;
}

String filename = System.mapLibraryName(libraryName);
List<String> candidates = new ArrayList<String>();
String lastError = null;
for (String directory : getLibPaths()) {
String candidate = directory + filename;
candidates.add(candidate);

if (IoUtils.canOpenReadOnly(candidate)) {
String error = doLoad(candidate, loader);
if (error == null) {
return; // We successfully loaded the library. Job done.
}
lastError = error;
}
}

if (lastError != null) {
throw new UnsatisfiedLinkError(lastError);
}
throw new UnsatisfiedLinkError("Library " + libraryName + " not found; tried " + candidates);
}

doLoad 方法

private String doLoad(String name, ClassLoader loader) {
// Android apps are forked from the zygote, so they can't have a custom LD_LIBRARY_PATH,
// which means that by default an app's shared library directory isn't on LD_LIBRARY_PATH.

// The PathClassLoader set up by frameworks/base knows the appropriate path, so we can load
// libraries with no dependencies just fine, but an app that has multiple libraries that
// depend on each other needed to load them in most-dependent-first order.

// We added API to Android's dynamic linker so we can update the library path used for
// the currently-running process. We pull the desired path out of the ClassLoader here
// and pass it to nativeLoad so that it can call the private dynamic linker API.

// We didn't just change frameworks/base to update the LD_LIBRARY_PATH once at the
// beginning because multiple apks can run in the same process and third party code can
// use its own BaseDexClassLoader.

// We didn't just add a dlopen_with_custom_LD_LIBRARY_PATH call because we wanted any
// dlopen(3) calls made from a .so's JNI_OnLoad to work too.

// So, find out what the native library search path is for the ClassLoader in question...
String librarySearchPath = null;
if (loader != null && loader instanceof BaseDexClassLoader) {
BaseDexClassLoader dexClassLoader = (BaseDexClassLoader) loader;
librarySearchPath = dexClassLoader.getLdLibraryPath();
}
// nativeLoad should be synchronized so there's only one LD_LIBRARY_PATH in use regardless
// of how many ClassLoaders are in the system, but dalvik doesn't support synchronized
// internal natives.
synchronized (this) {
// 这一边
return nativeLoad(name, loader, librarySearchPath);
}
}
// 这一边
// TODO: should be synchronized, but dalvik doesn't support synchronized internal natives.
private static native String nativeLoad(String filename, ClassLoader loader,
String librarySearchPath);

nativeLoad方法要去runtime.c（java_lang_Runtime.cc）

android-7.1.0_r1.7z\android-7.1.0_r\libcore\ojluni\src\main\native\runtime.c

runtime.c 这个类 在 7.0 里面是没有的，4.4 里面才有，所以要去 4.4 里面拷贝到7.0 源码里面

将4.4 源码 中的 dalvik/vm 拷贝到 7.0 的dalvik目录下

以下是 Runtime.c的源码

#include "jni.h"
#include "jni_util.h"
#include "jvm.h"

#include "JNIHelp.h"

#define NATIVE_METHOD(className, functionName, signature) \
{ #functionName, signature, (void*)(className ## _ ## functionName) }

JNIEXPORT jlong JNICALL
Runtime_freeMemory(JNIEnv *env, jobject this) {
return JVM_FreeMemory();
}

JNIEXPORT jlong JNICALL
Runtime_totalMemory(JNIEnv *env, jobject this) {
return JVM_TotalMemory();
}

JNIEXPORT jlong JNICALL
Runtime_maxMemory(JNIEnv *env, jobject this) {
return JVM_MaxMemory();
}

JNIEXPORT void JNICALL
Runtime_gc(JNIEnv *env, jobject this) {
JVM_GC();
}

JNIEXPORT void JNICALL
Runtime_nativeExit(JNIEnv *env, jclass this, jint status) {
JVM_Exit(status);
}

// 这个就是 nativeLoad 方法 的实现
JNIEXPORT jstring JNICALL
Runtime_nativeLoad(JNIEnv *env, jclass ignored, jstring javaFilename,
jobject javaLoader, jstring javaLibrarySearchPath) {
// JVM_NativeLoad 方法 在 OpenjdkJvm.cc 中
return JVM_NativeLoad(env, javaFilename, javaLoader, javaLibrarySearchPath);
}

static JNINativeMethod gMethods[] = {
// 使用了一个 NATIVE_METHOD 的 宏替换 ，这个宏替换在这个类的顶部
NATIVE_METHOD(Runtime, freeMemory, "!()J"),
NATIVE_METHOD(Runtime, totalMemory, "!()J"),
NATIVE_METHOD(Runtime, maxMemory, "!()J"),
NATIVE_METHOD(Runtime, gc, "()V"),
NATIVE_METHOD(Runtime, nativeExit, "(I)V"),
NATIVE_METHOD(Runtime, nativeLoad,
"(Ljava/lang/String;Ljava/lang/ClassLoader;Ljava/lang/String;)"
"Ljava/lang/String;"),
};

void register_java_lang_Runtime(JNIEnv *env) {
jniRegisterNativeMethods(env, "java/lang/Runtime", gMethods, NELEM(gMethods));
}

下面就是 OpenjdkJvm.cc

JNIEXPORT jstring JVM_NativeLoad(JNIEnv* env,
jstring javaFilename,
jobject javaLoader,
jstring javaLibrarySearchPath) {
ScopedUtfChars filename(env, javaFilename);
if (filename.c_str() == NULL) {
return NULL;
}

std::string error_msg;
{
// 这边 有一个 JavaVMExt  ， 这个方法的参数有一个 JNIEnv 。
// 那好，JavaVM* 和 JNIEnv 有什么区别呢？
// JavaVM* ： 一个android应用的进程，有且仅有一个javaVm
// JNIEnv ：每个java线程都对应一个env的环境变量
// 虚拟机里面jvm 是怎么找到具体的so库的堆栈的？，他调用了 JavaVM的loadNativeLibrary 方法里面，创建了一个结构体（这个结构体，包一个的指针，这个指针放我们真实加载完操作的文件地址），在这个结构体里面将我传进来的动态库()filename.c_str())加到结构体里面，然后保存到VM里面，那么对于我的android进程其他的地方，我只要拿到这个VM，就能找到这个结构体，通过这个结构体，就能找到这个so库里面的方法栈和引用内存
art::JavaVMExt* vm = art::Runtime::Current()->GetJavaVM();
// vm->LoadNativeLibrary 方法 在  java_vm_ext.cc
bool success = vm->LoadNativeLibrary(env,
filename.c_str(),
javaLoader,
javaLibrarySearchPath,
&error_msg);
if (success) {
return nullptr;
}
}

java_vm_ext.cc

bool JavaVMExt::LoadNativeLibrary(JNIEnv* env,
const std::string& path,
jobject class_loader,
jstring library_path,
std::string* error_msg) {
error_msg->clear();

// See if we've already loaded this library.  If we have, and the class loader
// matches, return successfully without doing anything.
// TODO: for better results we should canonicalize the pathname (or even compare
// inodes). This implementation is fine if everybody is using System.loadLibrary.
SharedLibrary* library;    // 创建SharedLibrary对象，SharedLibrary 是一个类对象
Thread* self = Thread::Current();
{
// TODO: move the locking (and more of this logic) into Libraries.
MutexLock mu(self, *Locks::jni_libraries_lock_);
library = libraries_->Get(path);// 实例化动态库library对象，这个path就是 so库的绝对路径，这个对象还没有赋值
}
void* class_loader_allocator = nullptr;
{
ScopedObjectAccess soa(env);
// As the incoming class loader is reachable/alive during the call of this function,
// it's okay to decode it without worrying about unexpectedly marking it alive.
mirror::ClassLoader* loader = soa.Decode<mirror::ClassLoader*>(class_loader);

ClassLinker* class_linker = Runtime::Current()->GetClassLinker(); // 获取ClassLinker对象
if (class_linker->IsBootClassLoader(soa, loader)) {
loader = nullptr;
class_loader = nullptr;
}

class_loader_allocator = class_linker->GetAllocatorForClassLoader(loader);
CHECK(class_loader_allocator != nullptr);
}
if (library != nullptr) {
// Use the allocator pointers for class loader equality to avoid unnecessary weak root decode.
if (library->GetClassLoaderAllocator() != class_loader_allocator) {
// The library will be associated with class_loader. The JNI
// spec says we can't load the same library into more than one
// class loader.
StringAppendF(error_msg, "Shared library \"%s\" already opened by "
"ClassLoader %p; can't open in ClassLoader %p",
path.c_str(), library->GetClassLoader(), class_loader);
LOG(WARNING) << error_msg;
return false;
}
VLOG(jni) << "[Shared library \"" << path << "\" already loaded in "
<< " ClassLoader " << class_loader << "]";
if (!library->CheckOnLoadResult()) {
StringAppendF(error_msg, "JNI_OnLoad failed on a previous attempt "
"to load \"%s\"", path.c_str());
return false;
}
return true;
}
// Open the shared library.  Because we're using a full path, the system
// doesn't have to search through LD_LIBRARY_PATH.  (It may do so to
// resolve this library's dependencies though.)

// Failures here are expected when java.library.path has several entries
// and we have to hunt for the lib.

// Below we dlopen but there is no paired dlclose, this would be necessary if we supported
// class unloading. Libraries will only be unloaded when the reference count (incremented by
// dlopen) becomes zero from dlclose.

Locks::mutator_lock_->AssertNotHeld(self);
const char* path_str = path.empty() ? nullptr : path.c_str();
// OpenNativeLibrary 是android 打开 natvie Library 并且返回 一个handle，这个handle赋值到了
// 这个handl 就是android 真实加载so库完之后返回的一个指针，这个handle指针放在SharedLibrary的对象library 中，而library 放到了 libraries_ 这个智能指针中，
void* handle = android::OpenNativeLibrary(env,
runtime_->GetTargetSdkVersion(),
path_str,
class_loader,
library_path);// 打开Native   库拿到一个handle 句柄

bool needs_native_bridge = false;
if (handle == nullptr) {
if (android::NativeBridgeIsSupported(path_str)) {
handle = android::NativeBridgeLoadLibrary(path_str, RTLD_NOW);
needs_native_bridge = true;
}
}

if (env->ExceptionCheck() == JNI_TRUE) {
LOG(ERROR) << "Unexpected exception:";
env->ExceptionDescribe();
env->ExceptionClear();
}
// Create a new entry.
// TODO: move the locking (and more of this logic) into Libraries.
bool created_library = false;
{
// Create SharedLibrary ahead of taking the libraries lock to maintain lock ordering.
// 这里用到一个 C++ 的智能指针 ，
std::unique_ptr<SharedLibrary> new_library(
// new SharedLibrary 的时候 传入了 handle 指针
new SharedLibrary(env, self, path, handle, class_loader, class_loader_allocator));
MutexLock mu(self, *Locks::jni_libraries_lock_);
library = libraries_->Get(path);
if (library == nullptr) {  // We won race to get libraries_lock.
library = new_library.release();
libraries_->Put(path, library);// 将我们指定的库加载进来，保存在library对象中
created_library = true;
}
}
if (!created_library) {
LOG(INFO) << "WOW: we lost a race to add shared library: "
<< "\"" << path << "\" ClassLoader=" << class_loader;
return library->CheckOnLoadResult();
}
VLOG(jni) << "[Added shared library \"" << path << "\" for ClassLoader " << class_loader << "]";

bool was_successful = false;
void* sym;
if (needs_native_bridge) {
library->SetNeedsNativeBridge();
}
sym = library->FindSymbol("JNI_OnLoad", nullptr);  // 拿到JNI_OnLoad方法
if (sym == nullptr) {
VLOG(jni) << "[No JNI_OnLoad found in \"" << path << "\"]";
was_successful = true;
} else {
// Call JNI_OnLoad.  We have to override the current class
// loader, which will always be "null" since the stuff at the
// top of the stack is around Runtime.loadLibrary().  (See
// the comments in the JNI FindClass function.)
ScopedLocalRef<jobject> old_class_loader(env, env->NewLocalRef(self->GetClassLoaderOverride()));
self->SetClassLoaderOverride(class_loader);

VLOG(jni) << "[Calling JNI_OnLoad in \"" << path << "\"]";
typedef int (*JNI_OnLoadFn)(JavaVM*, void*);
JNI_OnLoadFn jni_on_load = reinterpret_cast<JNI_OnLoadFn>(sym);
int version = (*jni_on_load)(this, nullptr);

if (runtime_->GetTargetSdkVersion() != 0 && runtime_->GetTargetSdkVersion() <= 21) {
fault_manager.EnsureArtActionInFrontOfSignalChain();
}

self->SetClassLoaderOverride(old_class_loader.get());

if (version == JNI_ERR) {
StringAppendF(error_msg, "JNI_ERR returned from JNI_OnLoad in \"%s\"", path.c_str());
} else if (IsBadJniVersion(version)) {
StringAppendF(error_msg, "Bad JNI version returned from JNI_OnLoad in \"%s\": %d",
path.c_str(), version);
// It's unwise to call dlclose() here, but we can mark it
// as bad and ensure that future load attempts will fail.
// We don't know how far JNI_OnLoad got, so there could
// be some partially-initialized stuff accessible through
// newly-registered native method calls.  We could try to
// unregister them, but that doesn't seem worthwhile.
} else {
// 加载成功的标志
was_successful = true;
}
VLOG(jni) << "[Returned " << (was_successful ? "successfully" : "failure")
<< " from JNI_OnLoad in \"" << path << "\"]";
}

library->SetResult(was_successful);
return was_successful;
}

static bool IsBadJniVersion(int version) {
// We don't support JNI_VERSION_1_1. These are the only other valid versions.
// 当不等于JNI_VERSION_1_2 或 JNI_VERSION_1_4  或 JNI_VERSION_1_6 就是个错误的version
return version != JNI_VERSION_1_2 && version != JNI_VERSION_1_4 && version != JNI_VERSION_1_6;
}

Java_vm_ext.h

// libraries_   是JVM 中的一个静态变量，有多少个so库，就会保存多少个SharedLibrary对象
std::unique_ptr<Libraries> libraries_  智能指针

关键是与JVM的联系：android进程，有且只有一个JavaVMExt*指针对象，当我们在LoadNativeLibrary的时候，new了一个SharedLibrary的对象指针，而SharedLibrary保存了handle句柄，然后在找文件方法的时候，都是通过对象里面的handle句柄来进行操作的，library有一个FindSymbol 来找方法，找到JNI_OnLoad方法去做具体的调用,这就是JNI设计的流程

6 . 用一个完整的例子来查看android是怎么实现动态注册的（MediaPlayer）

系统源码：frameworks\base\media\java\android\media\MediaPlayer.java

...
static {
System.loadLibrary("media_jni");
native_init();
}
...
private static native final void native_init();
private native final void native_setup(Object mediaplayer_this); // java函数名
private native final void native_finalize();

...

它的具体实现在 MediaPlayer.cpp里面

它的JNI的具体实现在 ./frameworks/base/media/jni/android_media_MediaPlayer.cpp

static JNINativeMethod gMethods[] = {
······
{"native_init",         "()V",                              (void *)android_media_MediaPlayer_native_init},
// 这边是 native_setup ： 第一个 是java函数名，第二个是签名，第三个是 jni具体实现方法的指针
{"native_setup",        "(Ljava/lang/Object;)V",            (void *)android_media_MediaPlayer_native_setup},
{"native_finalize",     "()V",                              (void *)android_media_MediaPlayer_native_finalize},
······
};

// jni具体实现方法的指针
static void
android_media_MediaPlayer_native_setup(JNIEnv *env, jobject thiz, jobject weak_this)
{
ALOGV("native_setup");
sp<MediaPlayer> mp = new MediaPlayer();
if (mp == NULL) {
jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
return;
}

// create new listener and give it to MediaPlayer
sp<JNIMediaPlayerListener> listener = new JNIMediaPlayerListener(env, thiz, weak_this);
mp->setListener(listener);

// Stow our new C++ MediaPlayer in an opaque field in the Java object.
setMediaPlayer(env, thiz, mp);
}

// This function only registers the native methods
static int register_android_media_MediaPlayer(JNIEnv *env)
{
// gMethods 在这边被调用，系统可以拿到AndroidRuntime:，我们拿不到，只能分析，他注册的时候做了什么事情，
// 分析： env ，"android/media/MediaPlayer" 是MediaPlayer.java的包名+类名
// gMethods
// NELEM(gMethods)算这个结构体数组的占多少个字节，将这个大小放进去(是个宏定义，便于复用)
// # define NELEM(x) ((int)(sizeof(x) / sizeof((x)[0])))
// registerNativeMethods 具体实现在AndroidRuntime.cpp 具体见下一段代码
return AndroidRuntime::registerNativeMethods(env,
"android/media/MediaPlayer", gMethods, NELEM(gMethods));
}

//这边重写了jni.h声明的 JNI_OnLoad方法,在JNI_OnLoad中进行注册（register_android_media_MediaPlayer），在注册过程中，声明了一个gMethods的结构体数组，这里面写好了方法映射。而JNI_OnLoad的调用处，就是System.loadLibrary 的时候会走到这里，然后进行动态注册
jint JNI_OnLoad(JavaVM* vm, void* /* reserved */)
{
JNIEnv* env = NULL;
jint result = -1;

if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {
ALOGE("ERROR: GetEnv failed\n");
goto bail;
}
assert(env != NULL);

...
// register_android_media_MediaPlayer 在这边被调用
if (register_android_media_MediaPlayer(env) < 0) {
ALOGE("ERROR: MediaPlayer native registration failed\n");
goto bail;
}
...

/* success -- return valid version number */
result = JNI_VERSION_1_4;

bail:
return result;
}

/frameworks/base/core/jni/AndroidRuntime.cpp

/*static*/ int AndroidRuntime::registerNativeMethods(JNIEnv* env,
const char* className, const JNINativeMethod* gMethods, int numMethods)
{
// jniRegisterNativeMethods 是在JNIHelp.cpp 里面实现的
return jniRegisterNativeMethods(env, className, gMethods, numMethods);
}

/external/conscrypt/src/compat/native/JNIHelp.cpp

extern "C" int jniRegisterNativeMethods(C_JNIEnv* env, const char* className,
const JNINativeMethod* gMethods, int numMethods)
{
JNIEnv* e = reinterpret_cast<JNIEnv*>(env);

ALOGV("Registering %s's %d native methods...", className, numMethods);
// 这边是重点 ，findClass 的实现是 env->FindClass(className)
scoped_local_ref<jclass> c(env, findClass(env, className));
if (c.get() == NULL) {
char* msg;
asprintf(&msg, "Native registration unable to find class '%s'; aborting...", className);
e->FatalError(msg);
}
// env的注册
if ((*env)->RegisterNatives(e, c.get(), gMethods, numMethods) < 0) {
char* msg;
asprintf(&msg, "RegisterNatives failed for '%s'; aborting...", className);
e->FatalError(msg);
}

return 0;
}

7 . JNI 动态注册

根据以上的分析进行实现

public class FileUtils {

public static native void diff(String path,String pattern_Path,int file_num);

public static void javaDiff(String path,String pattern_Path,int file_num){}

// Used to load the 'native-lib' library on application startup.
static {
System.loadLibrary("native-lib");
}}

——–JNI实现—————————-

#include "com_example_zeking_FileUtils.h"

#include <android/log.h>
#include <assert.h>

//int __android_log_print(int prio, const char* tag, const char* fmt, ...)
#define TAG "Zeking_JNI"
#define LOGI(...) __android_log_print(ANDROID_LOG_INFO, TAG, __VA_ARGS__)
# define NELEM(x) ((int) (sizeof(x) / sizeof((x)[0])))
/*
* Class:     com_example_zekign_FileUtils
* Method:    diff
* Signature: (Ljava/lang/String;Ljava/lang/String;I)V
*/
JNIEXPORT void JNICALL native_diff
(JNIEnv *env, jclass clazz, jstring path, jstring pattern_Path, jint file_num)
{

LOGI("JNI begin 动态注册的方法 ");

}

static const JNINativeMethod gMethods[] = {
{
"diff","(Ljava/lang/String;Ljava/lang/String;I)V",(void*)native_diff
}
};

static int registerNatives(JNIEnv* engv)
{
LOGI("registerNatives begin");
jclass  clazz;
clazz = (*engv) -> FindClass(engv, "com/example/zeking/FileUtils");

if (clazz == NULL) {
LOGI("clazz is null");
return JNI_FALSE;
}

if ((*engv) ->RegisterNatives(engv, clazz, gMethods, NELEM(gMethods)) < 0) {
LOGI("RegisterNatives error");
return JNI_FALSE;
}

return JNI_TRUE;
}

JNIEXPORT jint JNI_OnLoad(JavaVM* vm, void* reserved)
{

LOGI("jni_OnLoad begin");

JNIEnv* env = NULL;
jint result = -1;

if ((*vm)->GetEnv(vm,(void**) &env, JNI_VERSION_1_4) != JNI_OK) {
LOGI("ERROR: GetEnv failed\n");
return -1;
}
assert(env != NULL);

registerNatives(env);

return JNI_VERSION_1_4;
}

最后总结两者的区别：

静态注册：
每个class都需要使用javah生成一个头文件，并且生成的名字很长书写不便；初次调用时需要依据名字搜索对应的
JNI层函数来建立关联关系，会影响运行效率
用javah 生成头文件方便简单
javah生一个头文件 操作简单
名字很长 书写不方便
初次调用的使用，需要依据名字搜索对应的FindSymbol（具体看Runctime.c）
来找到对应的方法，如果方法数较多的时候，效率不高

动态注册：
第一次调用效率高
使用一种数据结构JNINativeMethod来记录java native函数和JNI函数的对应关系
移植方便，便于维护（一个java文件中有多个native方法，只要修改下gMethods 的映射关系）