介绍NDK平台都有哪些工具.通过NDK这套工具做安卓下的JNI开发.
可能有一些需求更适合通过C去做,有一些功能要通过C去实现.一个安卓程序,它本身还是一个Java应用.有一些功能/方法不通过Java来实现了,这个方法要通过JNI去调C的代码.
实际上安卓系统就是这么搞的.有JNI,Java才能调C/C++的函数.如果没有JNI,Java与C/C++之间沟通不了.所以JNI层安卓已经帮咱们实现好了.所以咱们实际上咱们在写这个代码的时候,包括放一个音乐,看这个视频都用到了JNI的技术.只不过人家封装好了,你不知道而已.既然人家都封装好了,那咱们还学什么呢？就直接用就完了嘛.它的这些C/C++开发的开源库不见得就会满足你的需求,可能安卓提供的这些C/C++的库不能满足你的需求.我现在想做一个特殊的加密逻辑,我想就放在C/C++开发的开源库,它安卓还没提供呢.那你就得自己写C/C++的代码.比如你想加一个人脸识别的需求,安卓目前没有人脸识别的功能.那你就得自己去搞C/C++的方法.搞C/C++的函数,然后通过JNI帮助咱们调到编译好的C/C++的模块.
写代码：
需求：通过点击一个按钮来调用C/C++的函数.
/* * Copyright (C) 2009 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package com.example.hellojni; import android.app.Activity; import android.widget.TextView; import android.os.Bundle; public class HelloJni extends Activity { /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); /* Create a TextView and set its content. * the text is retrieved by calling a native * function. */ TextView tv = new TextView(this); tv.setText( stringFromJNI() ); setContentView(tv); } /* A native method that is implemented by the * 'hello-jni' native library, which is packaged * with this application. */ public native String stringFromJNI(); /* This is another native method declaration that is *not* * implemented by 'hello-jni'. This is simply to show that * you can declare as many native methods in your Java code * as you want, their implementation is searched in the * currently loaded native libraries only the first time * you call them. * * Trying to call this function will result in a * java.lang.UnsatisfiedLinkError exception ! */ public native String unimplementedStringFromJNI(); /* this is used to load the 'hello-jni' library on application * startup. The library has already been unpacked into * /data/data/com.example.hellojni/lib/libhello-jni.so at * installation time by the package manager. */ static { System.loadLibrary("hello-jni"); } }
/* A native method that is implemented by the * 'hello-jni' native library, which is packaged * with this application. */ public native String stringFromJNI();
本地方法是由一个叫hello-jni的native函数库来实现的,放在这个应用当中的.咱们最终写的这个C的代码,会打包生成一个native library,最终的实现都会放在这个native library里.
NDK的samples是如何实现的?
/* * Copyright (C) 2009 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. * */ #include <string.h> #include <jni.h> /* This is a trivial JNI example where we use a native method * to return a new VM String. See the corresponding Java source * file located at: * * apps/samples/hello-jni/project/src/com/example/hellojni/HelloJni.java */ jstring Java_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env, jobject thiz ) { #if defined(__arm__) #if defined(__ARM_ARCH_7A__) #if defined(__ARM_NEON__) #define ABI "armeabi-v7a/NEON" #else #define ABI "armeabi-v7a" #endif #else #define ABI "armeabi" #endif #elif defined(__i386__) #define ABI "x86" #elif defined(__mips__) #define ABI "mips" #else #define ABI "unknown" #endif return (*env)->NewStringUTF(env, "Hello from JNI ! Compiled with ABI " ABI "."); }
查看jni.h
/* * Reference types, in C. */ typedef void* jobject; typedef jobject jclass; typedef jobject jstring; typedef jobject jarray; typedef jarray jobjectArray; typedef jarray jbooleanArray; typedef jarray jbyteArray; typedef jarray jcharArray; typedef jarray jshortArray; typedef jarray jintArray; typedef jarray jlongArray; typedef jarray jfloatArray; typedef jarray jdoubleArray; typedef jobject jthrowable; typedef jobject jweak;
jstring是jobject,jobject是void*.jobject它对应成一个任意类型的指针.Java的String它是一个对象,它并不是一个基本数据类型,所以它是存在在堆里面的.对一个Java的对象来讲,实际上在C里面是找不到这种对应的数据类型可以给它表示起来.但是因为jstring它是占据了这么一块连续的堆内存,所以说呢它就用一个任意类型的指针来表示这一个Java的字符串.只不过通过typedef给它起了一个别名,别名就叫做jstring.一看jstring就知道它对应Java的String类型.
虽说natvie方法没传任何的参数
/* A native method that is implemented by the * 'hello-jni' native library, which is packaged * with this application. */ public native String stringFromJNI();
但是到了native方法实现就多了两个必传的参数
jstring Java_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env, jobject thiz )
第二个参数jobject还是void*.只要是C基本数据类型它都是void*.jobject只是一个别名,用来标记它是对应Java的Object这个类型.
/* * Reference types, in C. */ typedef void* jobject;
由哪一个Java类/对象调用的这个native方法,最终在C这边它就会作为一个参数jobject传进来.
由于helloInC()是非静态的,是一个实例方法,必须得通过一个Java对象来进行调用.当前这个helloInC()方法是放在MainActivity里面,所以说你想运行它必须得保证一个MainActivity的实例才能调用这个helloInC().
public native String helloInC();
对于当前这个实例hello-jni来讲,jobject thiz是hello-jni.对于咱们要写的,jobject thiz是MainActivity.谁调用的native方法,最终这个jobject thiz就是谁.
第一个参数JNIEnv* env,Env,之前获取SD卡的时候接触过这个词,Environment,环境的意思.实际上一个JNIEnv*就是一个JNI环境.jni.h里面也可以找到JNIEnv
#else typedef const struct JNINativeInterface* JNIEnv; typedef const struct JNIInvokeInterface* JavaVM; #endif
typedef自定义类型,它的类型是struct JNINativeInterface*.struct JNINativeInterface是一个结构体,后面多了一个*,说明它struct JNINativeInterface*是这个结构体的一级指针.struct JNINativeInterface是一个结构体,struct JNINativeInterface*是一个一级指针.一级指针struct JNINativeInterface*的别名是JNIEnv.
JNIEnv* env
JNIEnv*又加了一个星,说明env是结构体struct JNINativeInterface的二级指针.
接口函数指针的表
/* * Table of interface function pointers. */ struct JNINativeInterface { void* reserved0; void* reserved1; void* reserved2; void* reserved3; jint (*GetVersion)(JNIEnv *); jclass (*DefineClass)(JNIEnv*, const char*, jobject, const jbyte*, jsize); jclass (*FindClass)(JNIEnv*, const char*); jmethodID (*FromReflectedMethod)(JNIEnv*, jobject); jfieldID (*FromReflectedField)(JNIEnv*, jobject); /* spec doesn't show jboolean parameter */ jobject (*ToReflectedMethod)(JNIEnv*, jclass, jmethodID, jboolean); jclass (*GetSuperclass)(JNIEnv*, jclass); jboolean (*IsAssignableFrom)(JNIEnv*, jclass, jclass); /* spec doesn't show jboolean parameter */ jobject (*ToReflectedField)(JNIEnv*, jclass, jfieldID, jboolean); jint (*Throw)(JNIEnv*, jthrowable); jint (*ThrowNew)(JNIEnv *, jclass, const char *); jthrowable (*ExceptionOccurred)(JNIEnv*); void (*ExceptionDescribe)(JNIEnv*); void (*ExceptionClear)(JNIEnv*); void (*FatalError)(JNIEnv*, const char*); jint (*PushLocalFrame)(JNIEnv*, jint); jobject (*PopLocalFrame)(JNIEnv*, jobject); jobject (*NewGlobalRef)(JNIEnv*, jobject); void (*DeleteGlobalRef)(JNIEnv*, jobject); void (*DeleteLocalRef)(JNIEnv*, jobject); jboolean (*IsSameObject)(JNIEnv*, jobject, jobject); jobject (*NewLocalRef)(JNIEnv*, jobject); jint (*EnsureLocalCapacity)(JNIEnv*, jint); jobject (*AllocObject)(JNIEnv*, jclass); jobject (*NewObject)(JNIEnv*, jclass, jmethodID, ...); jobject (*NewObjectV)(JNIEnv*, jclass, jmethodID, va_list); jobject (*NewObjectA)(JNIEnv*, jclass, jmethodID, jvalue*); jclass (*GetObjectClass)(JNIEnv*, jobject); jboolean (*IsInstanceOf)(JNIEnv*, jobject, jclass); jmethodID (*GetMethodID)(JNIEnv*, jclass, const char*, const char*); jobject (*CallObjectMethod)(JNIEnv*, jobject, jmethodID, ...); jobject (*CallObjectMethodV)(JNIEnv*, jobject, jmethodID, va_list); jobject (*CallObjectMethodA)(JNIEnv*, jobject, jmethodID, jvalue*); jboolean (*CallBooleanMethod)(JNIEnv*, jobject, jmethodID, ...); jboolean (*CallBooleanMethodV)(JNIEnv*, jobject, jmethodID, va_list); jboolean (*CallBooleanMethodA)(JNIEnv*, jobject, jmethodID, jvalue*); jbyte (*CallByteMethod)(JNIEnv*, jobject, jmethodID, ...); jbyte (*CallByteMethodV)(JNIEnv*, jobject, jmethodID, va_list); jbyte (*CallByteMethodA)(JNIEnv*, jobject, jmethodID, jvalue*); jchar (*CallCharMethod)(JNIEnv*, jobject, jmethodID, ...); jchar (*CallCharMethodV)(JNIEnv*, jobject, jmethodID, va_list); jchar (*CallCharMethodA)(JNIEnv*, jobject, jmethodID, jvalue*); jshort (*CallShortMethod)(JNIEnv*, jobject, jmethodID, ...); jshort (*CallShortMethodV)(JNIEnv*, jobject, jmethodID, va_list); jshort (*CallShortMethodA)(JNIEnv*, jobject, jmethodID, jvalue*); jint (*CallIntMethod)(JNIEnv*, jobject, jmethodID, ...); jint (*CallIntMethodV)(JNIEnv*, jobject, jmethodID, va_list); jint (*CallIntMethodA)(JNIEnv*, jobject, jmethodID, jvalue*); jlong (*CallLongMethod)(JNIEnv*, jobject, jmethodID, ...); jlong (*CallLongMethodV)(JNIEnv*, jobject, jmethodID, va_list); jlong (*CallLongMethodA)(JNIEnv*, jobject, jmethodID, jvalue*); jfloat (*CallFloatMethod)(JNIEnv*, jobject, jmethodID, ...) __NDK_FPABI__; jfloat (*CallFloatMethodV)(JNIEnv*, jobject, jmethodID, va_list) __NDK_FPABI__; jfloat (*CallFloatMethodA)(JNIEnv*, jobject, jmethodID, jvalue*) __NDK_FPABI__; jdouble (*CallDoubleMethod)(JNIEnv*, jobject, jmethodID, ...) __NDK_FPABI__; jdouble (*CallDoubleMethodV)(JNIEnv*, jobject, jmethodID, va_list) __NDK_FPABI__; jdouble (*CallDoubleMethodA)(JNIEnv*, jobject, jmethodID, jvalue*) __NDK_FPABI__; void (*CallVoidMethod)(JNIEnv*, jobject, jmethodID, ...); void (*CallVoidMethodV)(JNIEnv*, jobject, jmethodID, va_list); void (*CallVoidMethodA)(JNIEnv*, jobject, jmethodID, jvalue*); jobject (*CallNonvirtualObjectMethod)(JNIEnv*, jobject, jclass, jmethodID, ...); jobject (*CallNonvirtualObjectMethodV)(JNIEnv*, jobject, jclass, jmethodID, va_list); jobject (*CallNonvirtualObjectMethodA)(JNIEnv*, jobject, jclass, jmethodID, jvalue*); jboolean (*CallNonvirtualBooleanMethod)(JNIEnv*, jobject, jclass, jmethodID, ...); jboolean (*CallNonvirtualBooleanMethodV)(JNIEnv*, jobject, jclass, jmethodID, va_list); jboolean (*CallNonvirtualBooleanMethodA)(JNIEnv*, jobject, jclass, jmethodID, jvalue*); jbyte (*CallNonvirtualByteMethod)(JNIEnv*, jobject, jclass, jmethodID, ...); jbyte (*CallNonvirtualByteMethodV)(JNIEnv*, jobject, jclass, jmethodID, va_list); jbyte (*CallNonvirtualByteMethodA)(JNIEnv*, jobject, jclass, jmethodID, jvalue*); jchar (*CallNonvirtualCharMethod)(JNIEnv*, jobject, jclass, jmethodID, ...); jchar (*CallNonvirtualCharMethodV)(JNIEnv*, jobject, jclass, jmethodID, va_list); jchar (*CallNonvirtualCharMethodA)(JNIEnv*, jobject, jclass, jmethodID, jvalue*); jshort (*CallNonvirtualShortMethod)(JNIEnv*, jobject, jclass, jmethodID, ...); jshort (*CallNonvirtualShortMethodV)(JNIEnv*, jobject, jclass, jmethodID, va_list); jshort (*CallNonvirtualShortMethodA)(JNIEnv*, jobject, jclass, jmethodID, jvalue*); jint (*CallNonvirtualIntMethod)(JNIEnv*, jobject, jclass, jmethodID, ...); jint (*CallNonvirtualIntMethodV)(JNIEnv*, jobject, jclass, jmethodID, va_list); jint (*CallNonvirtualIntMethodA)(JNIEnv*, jobject, jclass, jmethodID, jvalue*); jlong (*CallNonvirtualLongMethod)(JNIEnv*, jobject, jclass, jmethodID, ...); jlong (*CallNonvirtualLongMethodV)(JNIEnv*, jobject, jclass, jmethodID, va_list); jlong (*CallNonvirtualLongMethodA)(JNIEnv*, jobject, jclass, jmethodID, jvalue*); jfloat (*CallNonvirtualFloatMethod)(JNIEnv*, jobject, jclass, jmethodID, ...) __NDK_FPABI__; jfloat (*CallNonvirtualFloatMethodV)(JNIEnv*, jobject, jclass, jmethodID, va_list) __NDK_FPABI__; jfloat (*CallNonvirtualFloatMethodA)(JNIEnv*, jobject, jclass, jmethodID, jvalue*) __NDK_FPABI__; jdouble (*CallNonvirtualDoubleMethod)(JNIEnv*, jobject, jclass, jmethodID, ...) __NDK_FPABI__; jdouble (*CallNonvirtualDoubleMethodV)(JNIEnv*, jobject, jclass, jmethodID, va_list) __NDK_FPABI__; jdouble (*CallNonvirtualDoubleMethodA)(JNIEnv*, jobject, jclass, jmethodID, jvalue*) __NDK_FPABI__; void (*CallNonvirtualVoidMethod)(JNIEnv*, jobject, jclass, jmethodID, ...); void (*CallNonvirtualVoidMethodV)(JNIEnv*, jobject, jclass, jmethodID, va_list); void (*CallNonvirtualVoidMethodA)(JNIEnv*, jobject, jclass, jmethodID, jvalue*); jfieldID (*GetFieldID)(JNIEnv*, jclass, const char*, const char*); jobject (*GetObjectField)(JNIEnv*, jobject, jfieldID); jboolean (*GetBooleanField)(JNIEnv*, jobject, jfieldID); jbyte (*GetByteField)(JNIEnv*, jobject, jfieldID); jchar (*GetCharField)(JNIEnv*, jobject, jfieldID); jshort (*GetShortField)(JNIEnv*, jobject, jfieldID); jint (*GetIntField)(JNIEnv*, jobject, jfieldID); jlong (*GetLongField)(JNIEnv*, jobject, jfieldID); jfloat (*GetFloatField)(JNIEnv*, jobject, jfieldID) __NDK_FPABI__; jdouble (*GetDoubleField)(JNIEnv*, jobject, jfieldID) __NDK_FPABI__; void (*SetObjectField)(JNIEnv*, jobject, jfieldID, jobject); void (*SetBooleanField)(JNIEnv*, jobject, jfieldID, jboolean); void (*SetByteField)(JNIEnv*, jobject, jfieldID, jbyte); void (*SetCharField)(JNIEnv*, jobject, jfieldID, jchar); void (*SetShortField)(JNIEnv*, jobject, jfieldID, jshort); void (*SetIntField)(JNIEnv*, jobject, jfieldID, jint); void (*SetLongField)(JNIEnv*, jobject, jfieldID, jlong); void (*SetFloatField)(JNIEnv*, jobject, jfieldID, jfloat) __NDK_FPABI__; void (*SetDoubleField)(JNIEnv*, jobject, jfieldID, jdouble) __NDK_FPABI__; jmethodID (*GetStaticMethodID)(JNIEnv*, jclass, const char*, const char*); jobject (*CallStaticObjectMethod)(JNIEnv*, jclass, jmethodID, ...); jobject (*CallStaticObjectMethodV)(JNIEnv*, jclass, jmethodID, va_list); jobject (*CallStaticObjectMethodA)(JNIEnv*, jclass, jmethodID, jvalue*); jboolean (*CallStaticBooleanMethod)(JNIEnv*, jclass, jmethodID, ...); jboolean (*CallStaticBooleanMethodV)(JNIEnv*, jclass, jmethodID, va_list); jboolean (*CallStaticBooleanMethodA)(JNIEnv*, jclass, jmethodID, jvalue*); jbyte (*CallStaticByteMethod)(JNIEnv*, jclass, jmethodID, ...); jbyte (*CallStaticByteMethodV)(JNIEnv*, jclass, jmethodID, va_list); jbyte (*CallStaticByteMethodA)(JNIEnv*, jclass, jmethodID, jvalue*); jchar (*CallStaticCharMethod)(JNIEnv*, jclass, jmethodID, ...); jchar (*CallStaticCharMethodV)(JNIEnv*, jclass, jmethodID, va_list); jchar (*CallStaticCharMethodA)(JNIEnv*, jclass, jmethodID, jvalue*); jshort (*CallStaticShortMethod)(JNIEnv*, jclass, jmethodID, ...); jshort (*CallStaticShortMethodV)(JNIEnv*, jclass, jmethodID, va_list); jshort (*CallStaticShortMethodA)(JNIEnv*, jclass, jmethodID, jvalue*); jint (*CallStaticIntMethod)(JNIEnv*, jclass, jmethodID, ...); jint (*CallStaticIntMethodV)(JNIEnv*, jclass, jmethodID, va_list); jint (*CallStaticIntMethodA)(JNIEnv*, jclass, jmethodID, jvalue*); jlong (*CallStaticLongMethod)(JNIEnv*, jclass, jmethodID, ...); jlong (*CallStaticLongMethodV)(JNIEnv*, jclass, jmethodID, va_list); jlong (*CallStaticLongMethodA)(JNIEnv*, jclass, jmethodID, jvalue*); jfloat (*CallStaticFloatMethod)(JNIEnv*, jclass, jmethodID, ...) __NDK_FPABI__; jfloat (*CallStaticFloatMethodV)(JNIEnv*, jclass, jmethodID, va_list) __NDK_FPABI__; jfloat (*CallStaticFloatMethodA)(JNIEnv*, jclass, jmethodID, jvalue*) __NDK_FPABI__; jdouble (*CallStaticDoubleMethod)(JNIEnv*, jclass, jmethodID, ...) __NDK_FPABI__; jdouble (*CallStaticDoubleMethodV)(JNIEnv*, jclass, jmethodID, va_list) __NDK_FPABI__; jdouble (*CallStaticDoubleMethodA)(JNIEnv*, jclass, jmethodID, jvalue*) __NDK_FPABI__; void (*CallStaticVoidMethod)(JNIEnv*, jclass, jmethodID, ...); void (*CallStaticVoidMethodV)(JNIEnv*, jclass, jmethodID, va_list); void (*CallStaticVoidMethodA)(JNIEnv*, jclass, jmethodID, jvalue*); jfieldID (*GetStaticFieldID)(JNIEnv*, jclass, const char*, const char*); jobject (*GetStaticObjectField)(JNIEnv*, jclass, jfieldID); jboolean (*GetStaticBooleanField)(JNIEnv*, jclass, jfieldID); jbyte (*GetStaticByteField)(JNIEnv*, jclass, jfieldID); jchar (*GetStaticCharField)(JNIEnv*, jclass, jfieldID); jshort (*GetStaticShortField)(JNIEnv*, jclass, jfieldID); jint (*GetStaticIntField)(JNIEnv*, jclass, jfieldID); jlong (*GetStaticLongField)(JNIEnv*, jclass, jfieldID); jfloat (*GetStaticFloatField)(JNIEnv*, jclass, jfieldID) __NDK_FPABI__; jdouble (*GetStaticDoubleField)(JNIEnv*, jclass, jfieldID) __NDK_FPABI__; void (*SetStaticObjectField)(JNIEnv*, jclass, jfieldID, jobject); void (*SetStaticBooleanField)(JNIEnv*, jclass, jfieldID, jboolean); void (*SetStaticByteField)(JNIEnv*, jclass, jfieldID, jbyte); void (*SetStaticCharField)(JNIEnv*, jclass, jfieldID, jchar); void (*SetStaticShortField)(JNIEnv*, jclass, jfieldID, jshort); void (*SetStaticIntField)(JNIEnv*, jclass, jfieldID, jint); void (*SetStaticLongField)(JNIEnv*, jclass, jfieldID, jlong); void (*SetStaticFloatField)(JNIEnv*, jclass, jfieldID, jfloat) __NDK_FPABI__; void (*SetStaticDoubleField)(JNIEnv*, jclass, jfieldID, jdouble) __NDK_FPABI__; jstring (*NewString)(JNIEnv*, const jchar*, jsize); jsize (*GetStringLength)(JNIEnv*, jstring); const jchar* (*GetStringChars)(JNIEnv*, jstring, jboolean*); void (*ReleaseStringChars)(JNIEnv*, jstring, const jchar*); jstring (*NewStringUTF)(JNIEnv*, const char*); jsize (*GetStringUTFLength)(JNIEnv*, jstring); /* JNI spec says this returns const jbyte*, but that's inconsistent */ const char* (*GetStringUTFChars)(JNIEnv*, jstring, jboolean*); void (*ReleaseStringUTFChars)(JNIEnv*, jstring, const char*); jsize (*GetArrayLength)(JNIEnv*, jarray); jobjectArray (*NewObjectArray)(JNIEnv*, jsize, jclass, jobject); jobject (*GetObjectArrayElement)(JNIEnv*, jobjectArray, jsize); void (*SetObjectArrayElement)(JNIEnv*, jobjectArray, jsize, jobject); jbooleanArray (*NewBooleanArray)(JNIEnv*, jsize); jbyteArray (*NewByteArray)(JNIEnv*, jsize); jcharArray (*NewCharArray)(JNIEnv*, jsize); jshortArray (*NewShortArray)(JNIEnv*, jsize); jintArray (*NewIntArray)(JNIEnv*, jsize); jlongArray (*NewLongArray)(JNIEnv*, jsize); jfloatArray (*NewFloatArray)(JNIEnv*, jsize); jdoubleArray (*NewDoubleArray)(JNIEnv*, jsize); jboolean* (*GetBooleanArrayElements)(JNIEnv*, jbooleanArray, jboolean*); jbyte* (*GetByteArrayElements)(JNIEnv*, jbyteArray, jboolean*); jchar* (*GetCharArrayElements)(JNIEnv*, jcharArray, jboolean*); jshort* (*GetShortArrayElements)(JNIEnv*, jshortArray, jboolean*); jint* (*GetIntArrayElements)(JNIEnv*, jintArray, jboolean*); jlong* (*GetLongArrayElements)(JNIEnv*, jlongArray, jboolean*); jfloat* (*GetFloatArrayElements)(JNIEnv*, jfloatArray, jboolean*); jdouble* (*GetDoubleArrayElements)(JNIEnv*, jdoubleArray, jboolean*); void (*ReleaseBooleanArrayElements)(JNIEnv*, jbooleanArray, jboolean*, jint); void (*ReleaseByteArrayElements)(JNIEnv*, jbyteArray, jbyte*, jint); void (*ReleaseCharArrayElements)(JNIEnv*, jcharArray, jchar*, jint); void (*ReleaseShortArrayElements)(JNIEnv*, jshortArray, jshort*, jint); void (*ReleaseIntArrayElements)(JNIEnv*, jintArray, jint*, jint); void (*ReleaseLongArrayElements)(JNIEnv*, jlongArray, jlong*, jint); void (*ReleaseFloatArrayElements)(JNIEnv*, jfloatArray, jfloat*, jint); void (*ReleaseDoubleArrayElements)(JNIEnv*, jdoubleArray, jdouble*, jint); void (*GetBooleanArrayRegion)(JNIEnv*, jbooleanArray, jsize, jsize, jboolean*); void (*GetByteArrayRegion)(JNIEnv*, jbyteArray, jsize, jsize, jbyte*); void (*GetCharArrayRegion)(JNIEnv*, jcharArray, jsize, jsize, jchar*); void (*GetShortArrayRegion)(JNIEnv*, jshortArray, jsize, jsize, jshort*); void (*GetIntArrayRegion)(JNIEnv*, jintArray, jsize, jsize, jint*); void (*GetLongArrayRegion)(JNIEnv*, jlongArray, jsize, jsize, jlong*); void (*GetFloatArrayRegion)(JNIEnv*, jfloatArray, jsize, jsize, jfloat*); void (*GetDoubleArrayRegion)(JNIEnv*, jdoubleArray, jsize, jsize, jdouble*); /* spec shows these without const; some jni.h do, some don't */ void (*SetBooleanArrayRegion)(JNIEnv*, jbooleanArray, jsize, jsize, const jboolean*); void (*SetByteArrayRegion)(JNIEnv*, jbyteArray, jsize, jsize, const jbyte*); void (*SetCharArrayRegion)(JNIEnv*, jcharArray, jsize, jsize, const jchar*); void (*SetShortArrayRegion)(JNIEnv*, jshortArray, jsize, jsize, const jshort*); void (*SetIntArrayRegion)(JNIEnv*, jintArray, jsize, jsize, const jint*); void (*SetLongArrayRegion)(JNIEnv*, jlongArray, jsize, jsize, const jlong*); void (*SetFloatArrayRegion)(JNIEnv*, jfloatArray, jsize, jsize, const jfloat*); void (*SetDoubleArrayRegion)(JNIEnv*, jdoubleArray, jsize, jsize, const jdouble*); jint (*RegisterNatives)(JNIEnv*, jclass, const JNINativeMethod*, jint); jint (*UnregisterNatives)(JNIEnv*, jclass); jint (*MonitorEnter)(JNIEnv*, jobject); jint (*MonitorExit)(JNIEnv*, jobject); jint (*GetJavaVM)(JNIEnv*, JavaVM**); void (*GetStringRegion)(JNIEnv*, jstring, jsize, jsize, jchar*); void (*GetStringUTFRegion)(JNIEnv*, jstring, jsize, jsize, char*); void* (*GetPrimitiveArrayCritical)(JNIEnv*, jarray, jboolean*); void (*ReleasePrimitiveArrayCritical)(JNIEnv*, jarray, void*, jint); const jchar* (*GetStringCritical)(JNIEnv*, jstring, jboolean*); void (*ReleaseStringCritical)(JNIEnv*, jstring, const jchar*); jweak (*NewWeakGlobalRef)(JNIEnv*, jobject); void (*DeleteWeakGlobalRef)(JNIEnv*, jweak); jboolean (*ExceptionCheck)(JNIEnv*); jobject (*NewDirectByteBuffer)(JNIEnv*, void*, jlong); void* (*GetDirectBufferAddress)(JNIEnv*, jobject); jlong (*GetDirectBufferCapacity)(JNIEnv*, jobject); /* added in JNI 1.6 */ jobjectRefType (*GetObjectRefType)(JNIEnv*, jobject); };
首先看看它struct JNINativeInterface声明的结构.
jint (*GetVersion)(JNIEnv *);
jint是返回值,GetVersion是函数指针的名字,JNIEnv *是形参的类型.
jint (*Throw)(JNIEnv*, jthrowable);
jint是返回值,Throw是函数指针的名字,JNIEnv*, jthrowable是形参的类型.
这里就定义了一堆函数指针,这些函数指针就可以帮助咱们做JNI的开发.
这些功能实际上都是通过JNINativeInterface这里面定义的这些函数指针来实现的.实际上就类似于一个函数表.这里面就可以调用到JNI开发常用的函数,通过函数指针来进行调用的.
C的源代码直接使用肯定是不行的,必须得把它编译.
#include <stdlib.h> #include <stdio.h> /*int main(int argc, char **argv) {//main是程序的入口. //但是今天java才是程序的入口,只不过是把一个方法通过C来实现.所以在C这边只需要写一个函数. //那这个函数是对我native方法的具体实现. //今天程序的入口是咱们的安卓应用.所以不能用main. } */ /*char* helloInC(){//人家java要的是String,你的是char *;你的helloInC怎么对java那边的helloInC(). //char *和String还是有点区别的. //java的String一定是丢在堆内存里面的,C的char *不一定丢在堆内存里面,有可能在栈里面. //所以实际上char *和String不是一种类型.所以这么写也不靠谱. } */ //本地函数命名规则 Java_包名_native函数所在类的类名_native方法名 //第二个参数jobject thiz 就是调用当前native方法的java对象. //第一个参数 JNIEnv* JNIEnv是结构体 JNINativeInterface这个结构体的一级指针 //env又是JNIEnv的一级指针 那么env就是JNINativeInterface的二级指针 //结构体JNINativeInterface定义了大量的函数指针 这些函数指针在JNI开发中十分常用. //这些功能实际上都是通过JNINativeInterface这里面定义的这些函数指针来实现的.实际上就类似于一个函数表.这里面就可以调用到JNI开发常用的函数,通过函数指针来进行调用的. //调用函数的第一种方法:(**env).func //第二种方式:(*env)-> //jstring Java_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env, // jobject thiz ) //jString是Java的String类型在C里面具体的表示的方式. jstring Java_com_itheima_hello_MainActivity_helloInC( JNIEnv* env, jobject thiz ){ //就得调用(*env)里面的函数指针实现相关的功能 //有没有哪一个函数指针它的返回值是一个jstring. // jstring (*NewStringUTF)(JNIEnv*, const char*); char* str ="hello from c!!!!!";//C的环境只能定义char* //得把char*转化成一个jstring类型才能返回.这些功能实际上都在这个JNIEnv所指向的这个JNINativeInterface里面. //通过newStringUTF 方法把C的字符串 转换成java的jstring类型. return (*env)->NewStringUTF(env,str);//(*env)->转化成一级指针调用 }
package com.itheima.hello; import android.os.Bundle; import android.app.Activity; import android.view.Menu; import android.view.View; import android.widget.Toast; public class MainActivity extends Activity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); } public void jniHello(View v){//Java不实现,只声明. //通过这个声明去调到C的方法.所以在Java这边去搞一个函数的声明. //调用恁地方法 并且把返回的值通过吐司显示出来 Toast.makeText(this, helloInC(), Toast.LENGTH_SHORT).show();//拿到C返回的字符串. } //public abstract String helloInC();//不能用抽象,abstract还不行. //用关键字native声明一个放在C这边的函数 通过native关键字声明了一个本地方法. //本地方法不用实现,需要用jni帝爱用C的代码来实现 public native String helloInC(); }
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:paddingBottom="@dimen/activity_vertical_margin" android:paddingLeft="@dimen/activity_horizontal_margin" android:paddingRight="@dimen/activity_horizontal_margin" android:paddingTop="@dimen/activity_vertical_margin" tools:context=".MainActivity" > <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:onClick="jniHello" android:text="调用C的函数" /> </RelativeLayout>