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12
xxx.c ——> windows .obj ； Linux .o –》 语法检查
2. 链接12
.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12345678
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);
}
}12345678910111213141516171819202122javah 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");
}
}1234567891011#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");
}12345678910111213141516171819jvm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
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 // 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"1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465666768697071724. 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");1System.java
public static void loadLibrary(String libname) {
Runtime.getRuntime().loadLibrary0(VMStack.getCallingClassLoader(), libname);
}123Runtime.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);
}123456789101112131415161718192021222324252627282930313233343536373839404142434445String filename = loader.findLibrary(libraryName);
点进去 发现是 return null；
ClassLoader.javaprotected String findLibrary(String libname) { return null;
}1234567891011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.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]]]1234
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 {
......
}123BaseDexClassLoader .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);
}@Overridepublic String findLibrary(String name) { return pathList.findLibrary(name);
}12345678910111213141516171819202122232425DexPathList .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;
}12345678910111213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);12345678910111213
再继续看 for (Element element : nativeLibraryPathElements) {
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);
......
}1234567891011121314151617181920212223245. 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);
}123456789101112131415161718192021222324252627282930313233343536373839404142434445doLoad 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);12345678910111213141516171819202122232425262728293031323334353637nativeLoad 方法 要去 runtime.c（java_lang_Runtime.cc）android-7.1.0_r1.7z\android-7.1.0_r1\libcore\ojluni\src\main\native\runtime.c
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));
}1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859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;
}
}123456789101112131415161718192021222324252627java_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;
}123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171Java_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12
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）
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();...123456789101112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 voidandroid_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 methodsstatic 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;
}12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596061626364656667/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);
}123456/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;
}123456789101112131415161718192021227. 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");
}
}1234567891011#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;
}12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596061626364656667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）
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