Android JNI 调用 C/C++ 接口
Android 使用 NDK 原生支持调用 c/c++ 接口的代码,只需要在程序中按照 android jni 规范编程就可以直接使用。
C 语言版本
JNI 调用 c 语言相对简单,命名一个 jni 函数,系统会自动注册到 Java 虚拟机,然后 Java 代码里面可以直接调用:
Native 代码:
#include <jni.h> int add(int x, int y) { return x + y; } jint Java_com_example_android_simplejni_SimpleJNI_add( JNIEnv* env, jobject this, jint x, jint y ) { return add(x, y);
需要注意的是函数名必须是跟 Java 类对应的,比如例子中的包为: package com.example.android.simplejni; 类名为: public class SimpleJNI
方法名为: add
然后 Java 类代码里面可以直接加载库,声明为自己的方法:
package com.example.android.simplejni; import android.app.Activity; import android.os.Bundle; import android.widget.TextView; public class SimpleJNI extends Activity { /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); TextView tv = new TextView(this); System.loadLibrary("simplejni"); int sum = add(2, 3); tv.setText("2 + 3 = " + Integer.toString(sum)); setContentView(tv); } public native int add(int x, int y); }
C++ 版本
c++ 语言由于跟 Java 一样是面向对象的设计语言,具有更多的特性,只是传递函数就不需要 c++ 了。 那么使用 C++ 代码注册到 Java 虚拟机就相对麻烦一些。
需要手动注册方法: 直接给 代码 吧:development/samples/SimpleJNI/jni/native.cpp
#define LOG_TAG "simplejni native.cpp" #include <utils/Log.h> #include <stdio.h> #include "jni.h" static jint add(JNIEnv *env, jobject thiz, jint a, jint b) { int result = a + b; ALOGI("%d + %d = %d", a, b, result); return result; } static const char *classPathName = "com/example/android/simplejni/Native"; static JNINativeMethod methods[] = { {"add", "(II)I", (void*)add }, }; /* * Register several native methods for one class. */ static int registerNativeMethods(JNIEnv* env, const char* className, JNINativeMethod* gMethods, int numMethods) { jclass clazz; clazz = env->FindClass(className); if (clazz == NULL) { ALOGE("Native registration unable to find class '%s'", className); return JNI_FALSE; } if (env->RegisterNatives(clazz, gMethods, numMethods) < 0) { ALOGE("RegisterNatives failed for '%s'", className); return JNI_FALSE; } return JNI_TRUE; } /* * Register native methods for all classes we know about. * * returns JNI_TRUE on success. */ static int registerNatives(JNIEnv* env) { if (!registerNativeMethods(env, classPathName, methods, sizeof(methods) / sizeof(methods[0]))) { return JNI_FALSE; } return JNI_TRUE; } // ---------------------------------------------------------------------------- /* * This is called by the VM when the shared library is first loaded. */ typedef union { JNIEnv* env; void* venv; } UnionJNIEnvToVoid; jint JNI_OnLoad(JavaVM* vm, void* reserved) { UnionJNIEnvToVoid uenv; uenv.venv = NULL; jint result = -1; JNIEnv* env = NULL; ALOGI("JNI_OnLoad"); if (vm->GetEnv(&uenv.venv, JNI_VERSION_1_4) != JNI_OK) { ALOGE("ERROR: GetEnv failed"); goto bail; } env = uenv.env; if (registerNatives(env) != JNI_TRUE) { ALOGE("ERROR: registerNatives failed"); goto bail; } result = JNI_VERSION_1_4; bail: return result; }
Java :SimpleJNI/src/com/example/android/simplejni/SimpleJNI.java
package com.example.android.simplejni; import android.app.Activity; import android.os.Bundle; import android.widget.TextView; public class SimpleJNI extends Activity { /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); TextView tv = new TextView(this); int sum = Native.add(2, 3); tv.setText("2 + 3 = " + Integer.toString(sum)); setContentView(tv); } } class Native { static { // The runtime will add "lib" on the front and ".o" on the end of // the name supplied to loadLibrary. System.loadLibrary("simplejni"); } static native int add(int a, int b); }
时间: 2024-10-12 12:23:25