Java Programming Tutorial-JNI(翻译)(续二)
? ? ? ? ? 最近太忙了，对这个JNI programing Tutorial的翻译停止了很长时间，正好赶上周末，把剩下的东西写下来，否则就虎头蛇尾了。
? ? ? ? ? JNI基础
? ? ? ? ? JNI在native系统定义了如下JNI的类型
? ? ? ? ? 1. java原始类型：jnit, jbyte, jshort,jlong,jfloat,jdouble,jchar,jboolean分别对应java中的int,byte,short,long,short,float,double,char 和boolean。
? ? ? ? ? 2. java引用类型：jobject对应java.lang.Object,定义了一下子类型：
? ? ? ? ? ? ? ? ? ? ? ?a. jclass ->java.lang.Class
? ? ? ? ? ? ? ? ? ? ? ?b. jstring->java.lang.String
? ? ? ? ? ? ? ? ? ? ? ?c. jthrowable->java.lang.Throwable
? ? ? ? ? ? ? ? ? ? ? ?d. jarray为java的数组，数组是引用类型可以存放8种原始类型和Object的数据，这8种原始类型的数组分别是:jintArray,jbyteArray,jshortArray,jlongArray,jfloatArray,jdoubleArray,jcharArray和jbooleanArray,还有救是jbojectArray.
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? ? ? ? ? native的函数可以接受这些JNI类型的参数并且返回这些类型的值，然而，native的函数只能操作native的数据类型，所以我们需要转换数据类型在native类型和JNI类型之间。转换的工作可能占到JNI代码的绝大多数。
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? ? ? ? ?传递原始类型数据
? ? ? ? ? Java JNI Program:TestJNIPrimitive.java
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public class TestJNIPrimitive { static { System.loadLibrary("myjni"); // myjni.dll (Windows) or libmyjni.so (Unixes) } // Native method that receives two ints and return a double containing the average private native double average(int n1, int n2); public static void main(String args[]) { System.out.println("In Java, the average is " + new TestJNIPrimitive().average(3, 2)); }}?
? ? ? ? ?这个java程序加载了一个共享库myjni.dll(在Windows上)或者libmyjni.so(在Unixes上). 声明了一个native的方法average(),接受两个int类型的参数，并且返回一个double类型的平均值。
? ? ? ? ?编译代码javac TestJNIPrimative.java，生成头文件javah TestJNIPrimitive,主要内容如下：
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JNIEXPORT jdouble JNICALL Java_TestJNIPrimitive_average(JNIEnv *, jobject, jint, jint);
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? ? ? ? ?“jni.h”和"win32\jni_mh.h"包含这8种JNI原始类型和一个jsize的typedef定义描述，如下：
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// In "win\jni_mh.h" - machine header which is machine dependenttypedef long jint;typedef __int64 jlong;typedef signed char jbyte; // In "jni.h"typedef unsigned char jboolean;typedef unsigned short jchar;typedef short jshort;typedef float jfloat;typedef double jdouble;typedef jint jsize;
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? ? ? ? ? ? ?实现的C文件TestJNIPrimitive.c如下：
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#include <jni.h>#include <stdio.h>#include "TestJNIPrimitive.h" JNIEXPORT jdouble JNICALL Java_TestJNIPrimitive_average (JNIEnv *env, jobject thisObj, jint n1, jint n2) { jdouble result; printf("In C, the numbers are %d and %d\n", n1, n2); result = ((jdouble)n1 + n2) / 2.0; // jint is mapped to int, jdouble is mapped to double return result;}?
? ? ? ? ? ? 编译：
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gcc -Wl,--add-stdcall-alias -I"<JAVA_HOME>\include" -I"<JAVA_HOME>\include\win32" -shared -o myjni.dll TestJNIPrimitive.c
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? ? ? ? ?传递String类型字符串：
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public class TestJNIString { static { System.loadLibrary("myjni"); // myjni.dll (Windows) or libmyjni.so (Unixes) } // Native method that receives a Java String and return a Java String private native String sayHello(String msg); public static void main(String args[]) { String result = new TestJNIString().sayHello("Hello from Java"); System.out.println("In Java, the returned string is: " + result); }}? ? ? ? 生成的头文件：
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JNIEXPORT jstring JNICALL Java_TestJNIString_sayHello(JNIEnv *, jobject, jstring);
? ? ? 传递和返回都是jstring类型，传递jstring类型要比传递原始类型要复杂，C的string是一个NULL结尾的字符数组，你需要在java的String类型和C的string类型做一个转换。
? ? ?转换步骤如下：
? ? ? 1. 使用方法char *GetStringUTFChars(JNIEnv*,jstring,jboolean)可以从jstring转换到C的string字符串。
? ? ? 2. NewStringUTF (JNIEnv*，char*)可以生产jstring类型字符串。
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#include <jni.h>#include <stdio.h>#include "TestJNIString.h" JNIEXPORT jstring JNICALL Java_TestJNIString_sayHello(JNIEnv *env, jobject thisObj, jstring inJNIStr) { // Step 1: Convert the JNI String (jstring) into C-String (char*) const char *inCStr = (*env)->GetStringUTFChars(env, inJNIStr, NULL); if (NULL == inCSt) return NULL; // Step 2: Perform its intended operations printf("In C, the received string is: %s\n", inCStr); (*env)->ReleaseStringUTFChars(env, inJNIStr, inCStr); // release resources // Prompt user for a C-string char outCStr[128]; printf("Enter a String: "); scanf("%s", outCStr); // not more than 127 characters // Step 3: Convert the C-string (char*) into JNI String (jstring) and return return (*env)->NewStringUTF(env, outCStr);}?
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? ? ? ? JNI native字符串的API：
? ? ? ? JNI支持Unicode(16位)和UTF-8(1~3字节)的字符串类型，像以null结尾的字符串utf - 8 C字符串(char数组),它应该被用于C / c++程序。
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// UTF-8 String (encoded to 1-3 byte, backward compatible with 7-bit ASCII)// Can be mapped to null-terminated char-array C-stringconst char * GetStringUTFChars(JNIEnv *env, jstring string, jboolean *isCopy); // Returns a pointer to an array of bytes representing the string in modified UTF-8 encoding.void ReleaseStringUTFChars(JNIEnv *env, jstring string, const char *utf); // Informs the VM that the native code no longer needs access to utf.jstring NewStringUTF(JNIEnv *env, const char *bytes); // Constructs a new java.lang.String object from an array of characters in modified UTF-8 encoding.jsize GetStringUTFLength(JNIEnv *env, jstring string); // Returns the length in bytes of the modified UTF-8 representation of a string.void GetStringUTFRegion(JNIEnv *env, jstring str, jsize start, jsize length, char *buf); // Translates len number of Unicode characters beginning at offset start into modified UTF-8 encoding // and place the result in the given buffer buf. // Unicode Strings (16-bit character)const jchar * GetStringChars(JNIEnv *env, jstring string, jboolean *isCopy); // Returns a pointer to the array of Unicode charactersvoid ReleaseStringChars(JNIEnv *env, jstring string, const jchar *chars); // Informs the VM that the native code no longer needs access to chars.jstring NewString(JNIEnv *env, const jchar *unicodeChars, jsize length); // Constructs a new java.lang.String object from an array of Unicode characters.jsize GetStringLength(JNIEnv *env, jstring string); // Returns the length (the count of Unicode characters) of a Java string.void GetStringRegion(JNIEnv *env, jstring str, jsize start, jsize length, jchar *buf); // Copies len number of Unicode characters beginning at offset start to the given buffer buf
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? ? ? ? ?传递原始类型的数组
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public class TestJNIPrimitiveArray { static { System.loadLibrary("myjni"); // myjni.dll (Windows) or libmyjni.so (Unixes) } // Native method that receives an int[] and // return a double[2] array with [0] as sum and [1] as average private native double[] sumAndAverage(int[] numbers); public static void main(String args[]) { int[] numbers = {22, 33, 33}; double[] results = new TestJNIPrimitiveArray().sumAndAverage(numbers); System.out.println("In Java, the sum is " + results[0]); System.out.println("In Java, the average is " + results[1]); }}? ? ?生成的头文件主要内容为：
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JNIEXPORT jdoubleArray JNICALL Java_TestJNIPrimitiveArray_average (JNIEnv *, jobject, jintArray);
? ? ?现在需要JNI数组和C语言数组中间做一个转换：
? ? ?1. 使用jnit *GetIntArrayElements(JNIEnv *env,jintArray a,jboolean *isCopy)，可以得到一个native的jint [].
? ? ?2. 使用jintArray NewIntArray(JNIEnv *env, jsize len)，可以生产一个JNI jintArray的对象。
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#include <jni.h>#include <stdio.h>#include "TestJNIPrimitiveArray.h" JNIEXPORT jdoubleArray JNICALL Java_TestJNIPrimitiveArray_sumAndAverage (JNIEnv *env, jobject thisObj, jintArray inJNIArray) { // Step 1: Convert the incoming JNI jintarray to C's jint[] jint *inCArray = (*env)->GetIntArrayElements(env, inJNIArray, NULL); if (NULL == inCArray) return NULL; jsize length = (*env)->GetArrayLength(env, inJNIArray); // Step 2: Perform its intended operations jint sum = 0; int i; for (i = 0; i < length; i++) { sum += inCArray[i]; } jdouble average = (jdouble)sum / length; (*env)->ReleaseIntArrayElements(env, inJNIArray, inCArray, 0); // release resources jdouble outCArray[] = {sum, average}; // Step 3: Convert the C's Native jdouble[] to JNI jdoublearray, and return jdoubleArray outJNIArray = (*env)->NewDoubleArray(env, 2); // allocate if (NULL == outJNIArray) return NULL; (*env)->SetDoubleArrayRegion(env, outJNIArray, 0 , 2, outCArray); // copy return outJNIArray;}? ? ?JNI Array API：
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// ArrayType: jintArray, jbyteArray, jshortArray, jlongArray, jfloatArray, jdoubleArray, jcharArray, jbooleanArray// PrimitiveType: int, byte, short, long, float, double, char, boolean// NativeType: jint, jbyte, jshort, jlong, jfloat, jdouble, jchar, jbooleanNativeType * Get<PrimitiveType>ArrayElements(JNIEnv *env, ArrayType array, jboolean *isCopy);void Release<PrimitiveType>ArrayElements(JNIEnv *env, ArrayType array, NativeType *elems, jint mode);void Get<PrimitiveType>ArrayRegion(JNIEnv *env, ArrayType array, jsize start, jsize length, NativeType *buffer);void Set<PrimitiveType>ArrayRegion(JNIEnv *env, ArrayType array, jsize start, jsize length, const NativeType *buffer);ArrayType New<PrimitiveType>Array(JNIEnv *env, jsize length);void * GetPrimitiveArrayCritical(JNIEnv *env, jarray array, jboolean *isCopy);void ReleasePrimitiveArrayCritical(JNIEnv *env, jarray array, void *carray, jint mode);
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? ? 未完待续。。。。
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