干货|JNI实现机制

methodID_toString = NULL;

}

//隐式注册

extern "C" JNIEXPORT jstring JNICALL

Java_com_mob_test_jni_testjni_JNIMethods_stringFromJNI(JNIEnv *env, jobject jobj) {

LOGD("test-jni.cpp Java_com_mob_test_jni_testjni_JNIMethods_stringFromJNI");

return stringFromJNIDis(env, jobj);

}

extern "C" JNIEXPORT jstring JNICALL

Java_com_mob_test_jni_testjni_JNIMethods_testParams(JNIEnv *env, jclass type, jint jint_, jstring jstring_, jobject jobject_,

jobjectArray arrayObject,

jobject list) {

LOGD("test-jni.cpp Java_com_mob_test_jni_testjni_JNIMethods_testParams");

return testParamsDis(env, type, jint_, jstring_, jobject_, arrayObject, list);

}

//显式注册

jstring stringFromJNIDis(JNIEnv *env, jobject jobj) {

LOGD("test-jni.cpp stringFromJNIDis");

std::string hello = "Hello from C++";

return env->NewStringUTF(hello.c_str());

}

jstring testParamsDis(JNIEnv *env, jclass type, jint jint_, jstring jstring_, jobject jobject_, jobjectArray arrayObject, jobject list) {

LOGD("test-jni.cpp testParamsDis");

char returnValue[1024];

int i = jint_;

sprintf(returnValue, "%d", i);

appendString(env, returnValue, jstring_);

jstring returnToString = reinterpret_cast<_jstring *>(env->CallObjectMethod(jobject_, methodID_toString));

appendString(env, returnValue, returnToString);

jsize len = env->GetArrayLength(arrayObject);

jobject tmpObject;

jstring tmpString;

for (i = 0; i < len; i++) {

tmpObject = env->GetObjectArrayElement(arrayObject, i);

tmpString = reinterpret_cast<_jstring *>(env->CallObjectMethod(tmpObject, methodID_toString));

appendString(env, returnValue, tmpString);

}

env->DeleteLocalRef(tmpObject);

jclass clazz_list = env->GetObjectClass(list);

jmethodID list_get = env->GetMethodID(clazz_list, "get", "(I)Ljava/lang/Object;");

jmethodID list_size = env->GetMethodID(clazz_list, "size", "()I");

int size = env->CallIntMethod(list, list_size);

jstring itemStr;

for (i = 0; i < size; i++) {

itemStr = reinterpret_cast(env->CallObjectMethod(list, list_get, i));

appendString(env, returnValue, itemStr);

}

return env->NewStringUTF(returnValue);

}

3. 在gradle中设置cmake编译选项，并编写CMakeLists.txt剧本，用于天生so

android {

externalNativeBuild {

cmake {

path "CMakeLists.txt"

}

}

}

#CMakeList.txt 内容如下：

# Sets the minimum version of CMake required to build the native library.

cmake_minimum_required(VERSION 3.4.1)

set(distribution_DIR ${CMAKE_SOURCE_DIR}/src/main/cpp)

add_library( # Sets the name of the library.

test-jni

# Sets the library as a shared library.

SHARED

# Provides a relative path to your source file(s).

src/main/cpp/test-jni.cpp)

find_library( # Sets the name of the path variable.

test

# Specifies the name of the NDK library that

# you want CMake to locate.

log)

target_link_libraries( # Specifies the target library.

test-jni

# Links the target library to the log library

# included in the NDK.

${log-lib} )

4. 在java层，加载当地要领前，加载so

public class JNIMethods {

static {

//加载so

System.loadLibrary("test-jni");

}

//隐式注册

public native String stringFromJNI();

public static native String testParams(int jint, String jstring, Object jobject, Object[] arrayObject, Listlist);

//显式注册

public native String stringFromJNIDis();

public static native String testParamsDis(int jint, String jstring, Object jobject, Object[] arrayObject, Listlist);

}

so加载流程

一样平常我们是通过 System.loadLibrary("test-jni") 的方式来加载libtest-jni.so库的，一起来跟踪下整个加载历程（此处剖析的源码是基于Dalvik的）

System.loadLibrary() -> Runtime.loadLibray0() -> Runtime.doLoad() -> Runtime.nativeLoad()

//java.lang.System

public static void loadLibrary(String libname) {

Runtime.getRuntime().loadLibrary0(VMStack.getCallingClassLoader(), libname);

}

//java.lang.Runtime

synchronized void loadLibrary0(ClassLoader loader, String libname) {

//......

String error = doLoad(filename, loader);

//......

}

private String doLoad(String name, ClassLoader loader) {

//......

synchronized (this) {

return nativeLoad(name, loader, librarySearchPath);

}

}

// TODO: should be synchronized, but dalvik doesn't support synchronized internal natives.

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