NDK开发之c++调用java

文章摘自：http://blog.csdn.net/lamp_zy/article/details/8218963

在NDK中创建的线程中, 只允许调用静态的Java API. 当在线程中调用env->FindClass(), 系统就会发出异常

JAVA 代码:

package com.myndk;

...

public class simple_test extends Activity {

...

public void PrintNdkLog(String slog) { // 成员函数, 不可在线程中被调用

Log.e("NDK", slog);

}

public static void PrintNdkLogThread(String slog) { // 静态函数, 可在主线中被调用

Log.e("NDK-THREAD", slog);

}

}

C++
代码:

static
JavaVM* g_JavaVM = NULL;

static jobject g_InterfaceObject = 0;

static const char *g_JavaClassName = "com/myndk/simple_test";

//传入class，输出这个class的一个object

static jobject getInstance(JNIEnv *env, jclass obj_class)

{

jmethodID c_id = env->GetMethodID(obj_class, "<init>", "()V");//得到构造函数，得到构造函数ID必须这样写

jobject obj = env->NewObject(obj_class, c_id);//new 一个对象，并调用构造函数

return obj;

}

//线程中无法调用FindClass，所有要在JniLoad中提前得到object

static void GetInterfaceObject(JNIEnv *env, const char *path, jobject *objptr)

{

jclass cls = env->FindClass(path);

if (!cls)

{

return;

}

jmethodID constr = env->GetMethodID(cls, "<init>", "()V");

if (!constr)

{

return;

}

jobject obj = env->NewObject(cls, constr);

if (!obj)

{

return;

}

(*objptr) = env->NewGlobalRef(obj);

}

//线程中无法调用FindClass，所有要在JniLoad中提前得到object

int JniLoad(JavaVM* jvm, void* reserved)

{

g_JavaVM = jvm;

JNIEnv *env;

if (jvm->GetEnv((void**) &env, JNI_VERSION_1_6) != JNI_OK)

{

return -1;

}

GetInterfaceObject(env, g_JavaClassName, &g_InterfaceObject);

return JNI_VERSION_1_6;

}

void JniUnLoad(JavaVM* jvm, void* reserved)

{

JNIEnv *env;

if (jvm->GetEnv((void**) &env, JNI_VERSION_1_6) != JNI_OK)

{

return;

}

env->DeleteGlobalRef(g_InterfaceObject);

}

void
PrintNdkLog(const char *format, ...)

{

va_list arg_ptr;

char sLogBuff[1024];

if (g_JavaVM == NULL)

return;

//把可变个数的参数一个个写到sLogBuff中

va_start(arg_ptr, format);

vsprintf(sLogBuff, format, arg_ptr);

va_end(arg_ptr);

int status;

JNIEnv *env = NULL;

bool isAttached = false;

status = g_JavaVM->GetEnv((void**) &env, JNI_VERSION_1_6);

//如果PrintNdkLog是在线程中调用

if (status < 0) //表示PrintNdkLog是在线程中被调用的

{

status = g_JavaVM->AttachCurrentThread(&env, NULL);

if (status < 0)

{

return;

}

isAttached = true;

}

if (isAttached) // PrintNdkLog在线程中被调用, 调用Java静态API

{

jclass cls = env->GetObjectClass(g_InterfaceObject);

if (cls != 0)

{

jmethodID mid = env->GetStaticMethodID(cls, "PrintNdkLogThread", "(Ljava/lang/String;)V");

if (mid != 0)

{

jstring jstrMSG = env->NewStringUTF(sLogBuff);

env->CallStaticVoidMethod(cls, mid, jstrMSG);

}

}

g_JavaVM->DetachCurrentThread();

}

else // PrintNdkLog不是在线程中被调用, 调用Java类成员API

{

jclass cls = env->FindClass(g_JavaClassName);

if (cls != 0)

{

jobject obj = getInstance(env, cls);

jmethodID mid = env->GetMethodID(cls, "PrintNdkLog", "(Ljava/lang/String;)V");

if (mid != 0)

{

jstring jstrMSG = env->NewStringUTF(sLogBuff);

env->CallVoidMethod(obj, mid, jstrMSG);

}

}

}

}