下面是幾個例子程序:
1、C++本地方法中獲取Java中的變量以及調用Java中的方法
Java代碼:
1 package com.test; 2 3 import java.util.Date; 4 5 public class TestNative { 6 public native void sayHello(); 7 private int a = 10; 8 public int function(int x,Date date,int[] y){ 9 System.out.println("function"); 10 return 0; 11 } 12 public double max(double a,double b){ 13 return a>b?a:b; 14 } 15 /** 16 * @param args 17 */ 18 public static void main(String[] args) { 19 System.loadLibrary("NativeCode"); 20 TestNative tNative = new TestNative(); 21 tNative.sayHello(); 22 } 23 24 }
C++本地代碼:
com_test_TestNative.h代碼省略了
1 #include<iostream.h> 2 #include"com_test_TestNative.h" 3 4 JNIEXPORT void JNICALL Java_com_test_TestNative_sayHello(JNIEnv *env, jobject obj) 5 { 6 //因爲sayHello不是靜態函數,所以傳進來的obj就是調用這個函數的對象,否則就是傳入native方法所在的類 7 jclass hello_clazz = env->GetObjectClass(obj); //得到的就是native方法所在的類 8 // jfieldID fieldId_a = env->GetFieldID(hello_clazz,"a","I"); 9 // jmethodID methodId_fun = env->GetMethodID(hello_clazz,"function","(ILjava/util/Date;[I)I"); 10 // env->CallIntMethod(obj,methodId_fun,0L,NULL,NULL); //方法調用 11 // cout<<"hello world!"<<endl; 12 // cout<<"successful"<<endl; 13 /* 14 jfieldID field_a = env->GetFieldID(hello_clazz,"a","I"); //得到字段a的ID 15 jint a = env->GetIntField(obj,field_a); //得到字段a的值 16 cout<<a<<endl; 17 env->SetIntField(obj,field_a,100L); 18 */ 19 jmethodID methodId_max = env->GetMethodID(hello_clazz,"max","(DD)D"); 20 jvalue *values = new jvalue[2]; //jvalue是用來向java函數中傳參數的 21 values[0].d = 3.14; 22 values[1].d = 3.22; 23 //jdouble max = env->CallDoubleMethod(obj,methodId_max,3.18,3.15); //第一種方法調用 24 jdouble max = env->CallDoubleMethodA(obj,methodId_max,values); //第二種方法調用 25 delete [] values; 26 cout<<max<<endl; 27 28 }
2、C++本地方法中對Java中的字符串操作(實現用戶輸入一個字符串,在C++中對其反轉)
Java代碼:
1 package com.test; 2 3 import java.io.BufferedReader; 4 import java.io.IOException; 5 import java.io.InputStreamReader; 6 7 public class TestStr { 8 public native void cppCode(); 9 private String message; 10 public static void main(String[] args) throws IOException { 11 System.loadLibrary("CPP"); 12 BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); 13 String str = reader.readLine(); 14 TestStr obj = new TestStr(); 15 obj.message = str; 16 obj.cppCode(); 17 System.out.println("java :"+obj.message); 18 } 19 20 }
c++本地方法:
com_test_TestStr.h省略了
1 #include "com_test_TestStr.h" 2 #include<windows.h> 3 #include<string> 4 #include<algorithm> 5 using namespace std; 6 JNIEXPORT void JNICALL Java_com_test_TestStr_cppCode(JNIEnv *env, jobject obj) 7 { 8 jfieldID fid_msg = env->GetFieldID(env->GetObjectClass(obj),"message","Ljava/lang/String;"); 9 jstring j_msg = (jstring)env->GetObjectField(obj,fid_msg); 10 jsize len = env->GetStringLength(j_msg); //得到字符串的長度 11 jchar* jstr = new jchar[len+1]; //申請空間 12 jstr[len] = L'\0'; 13 env->GetStringRegion(j_msg,0,len,jstr); 14 //env->ReleaseStringChars(j_msg,jstr); 15 //MessageBoxW(NULL,(const wchar_t*)jstr,L"Title",MB_OK); 16 wstring wstr((const wchar_t*)jstr); 17 delete[] jstr; //釋放空間 18 std::reverse(wstr.begin(),wstr.end()); //字符串反轉 19 jstring j_new_str = env->NewString((const jchar*)wstr.c_str(),(jint)wstr.size()); //創建一個新的字符串 20 env->SetObjectField(obj,fid_msg,j_new_str); //設置字符串給java對象 21 22 }
3、C++本地方法中獲取Java中的數組(實現用C++本地方法將Java中的數組排序)
Java代碼:
1 package com.test; 2 3 public class TestArray { 4 5 public int[] array = {5,7,3,4,1,9,2,8,6,0}; 6 7 public native void callCppFun(); 8 9 public static void main(String[] args) { 10 System.loadLibrary("TestArray"); 11 TestArray obj = new TestArray(); 12 obj.callCppFun(); 13 for(int a:obj.array){ 14 System.out.println(a); 15 } 16 } 17 18 }
C++本地代碼:
com_test_TestArray.h省略了
1 #include"com_test_TestArray.h" 2 #include<iostream> 3 #include<algorithm> 4 using namespace std; 5 JNIEXPORT void JNICALL Java_com_test_TestArray_callCppFun (JNIEnv *env, jobject obj) 6 { 7 jfieldID fid_array = env->GetFieldID(env->GetObjectClass(obj),"array","[I"); //獲取數組的id 8 jintArray jint_array = (jintArray)env->GetObjectField(obj,fid_array); 9 10 jint* int_arr = env->GetIntArrayElements(jint_array,NULL); //轉化爲jint數組 11 jsize len = env->GetArrayLength(jint_array); 12 std::sort(int_arr,int_arr+len); //對數組進行排序 13 env->ReleaseIntArrayElements(jint_array,int_arr,0); //0:釋放C++數組,並且更新到Java 14 /* for(jsize i=0;i<len;i++){ 15 cout<<int_arr[i]<<endl; 16 } 17 */ 18 // env->ReleaseIntArrayElements(jint_array,int_arr,JNI_ABORT);//JNI_ABORT:釋放C++數組,但是不更新到Java 19 }
- public class MainActivity extends Activity{
- static {
- System.loadLibrary("JniTest");
- }
- public native String getStringFromNative();
- @Override
- protected void onCreate(Bundle savedInstanceState) {
- super.onCreate(savedInstanceState);
- setContentView(R.layout.activity_main);
- TextView txtView = (TextView) findViewById(R.id.txt);
- txtView.setText(getStringFromNative());
- }
- }
Step: 2.執行Build->Make Project
Step: 3.javah生成c頭文件
- javah -d jni -classpath c:\Users\sodinochen\AppData\Local\Android\sdk\platforms
- \android-16\android.jar;..\..\build\intermediates\classes\debug lab.sodino.jnitest.MainActivity
最後的生成結果:
Step: 4.編輯c文件
- /* DO NOT EDIT THIS FILE - it is machine generated */
- #include <jni.h>
- #include <android/log.h>
- #ifndef LOG_TAG
- #define LOG_TAG "ANDROID_LAB"
- #define LOGE(...) __android_log_print(ANDROID_LOG_ERROR, LOG_TAG, __VA_ARGS__)
- #endif
- /* Header for class lab_sodino_jnitest_MainActivity */
- #ifndef _Included_lab_sodino_jnitest_MainActivity
- #define _Included_lab_sodino_jnitest_MainActivity
- #ifdef __cplusplus
- extern "C" {
- #endif
- /*
- * Class: lab_sodino_jnitest_MainActivity
- * Method: getStringFromNative
- * Signature: ()Ljava/lang/String;
- */
- JNIEXPORT jstring JNICALL Java_lab_sodino_jnitest_MainActivity_getStringFromNative
- (JNIEnv * env, jobject jObj){
- LOGE("log string from ndk.");
- return (*env)->NewStringUTF(env,"Hello From JNI!");
- }
- #ifdef __cplusplus
- }
- #endif
- #endif
- Error:Execution failed for task ':app:compileDebugNdk'.
- > NDK not configured.
- Download the NDK from http://developer.android.com/tools/sdk/ndk/.Then add ndk.dir=path/to/ndk in local.properties.
- (On Windows, make sure you escape backslashes, e.g. C:\\ndk rather than C:\ndk)
- ndk {
- moduleName "JniTest"
- ldLibs "log", "z", "m"
- abiFilters "armeabi", "armeabi-v7a", "x86"
- }
- Error:Execution failed for task ':app:compileDebugNdk'.
- > com.android.ide.common.internal.LoggedErrorException: Failed to run command:
- D:\Mission\adt-bundle-windows\ndk-r10b\ndk-build.cmd NDK_PROJECT_PATH=null APP_BUILD_SCRIPT=C:\Users\sodinochen\AndroidstudioProjects\JniTest2\app\build\intermediates\ndk\debug\Android.mk APP_PLATFORM=android-21 NDK_OUT=C:\Users\sodinochen\AndroidstudioProjects\JniTest2\app\build\intermediates\ndk\debug\obj NDK_LIBS_OUT=C:\Users\sodinochen\AndroidstudioProjects\JniTest2\app\build\intermediates\ndk\debug\lib APP_ABI=armeabi,armeabi-v7a,x86
- Error Code:
- 2
- Output:
- make.exe: *** No rule to make target `C:\Users\sodinochen\AndroidstudioProjects\JniTest2\app\build\intermediates\ndk\debug\obj/local/armeabi/objs/JniTest/C_\Users\sodinochen\AndroidstudioProjects\JniTest2\app\src\main\jni', needed by `C:\Users\sodinochen\AndroidstudioProjects\JniTest2\app\build\intermediates\ndk\debug\obj/local/armeabi/objs/JniTest/C_\Users\sodinochen\AndroidstudioProjects\JniTest2\app\src\main\jni\main.o'. Stop.
Step: 6.安裝運行
查看Log打印: