JNI基础介绍 - My Octopress Blog

这里简单介绍一下JNI的语法

初识第一面

HelloJNI.java

public class HelloJNI {
   static {
      System.loadLibrary("hello"); // Load native library at runtime
                                   // hello.dll (Windows) or libhello.so (Unixes)
   }

   // Declare a native method sayHello() that receives nothing and returns void
   private native void sayHello();

   // Test Driver
   public static void main(String[] args) {
      new HelloJNI().sayHello();  // invoke the native method
   }
}

HelloJNI.h/.c

JNIEXPORT void JNICALL Java_HelloJNI_sayHello(JNIEnv *env, jobject thisObj) {
   printf("Hello World!\n");
   return;
}

需要注意的有：

函数命名约定 Java_{package_and_classname}_{function_name}(JNI arguments)
JNIEnv*: 对于JNI环境的引用, 通过这个入口可以进行jni函数
jobject: 对于this对象的引用

JNI基础

JNI在native系统中定义了下面这些JNI数据类型，来对应Java的类型。

Java基本类型：jint，jbyte，jshort，jlong，jfloat，jdouble，jchar，jboolean对应Java的int，byte，short，long，float，double，char，boolean
Java应用类型：jobject对应java.lang.Object。同时还有一下子类型
jclass -> java.lang.Class
jstring -> java.lang.String
jthrowable -> java.lang.Throwable
jarray -> Java Array。也就是jintArray, jbyteArray, jshortArray, jlongArray, jfloatArray, jdoubleArray, jcharArray, jbooleanArray, jobjectArray.

由于native的函数都是接受JNI类型参数，返回JNI类型参数，所以一般情况下我们需要这样做：

把JNI的参数转换或者拷贝成C可以操作的类型，如jintArray到int[]
使用C类型参数完成计算
把结果转换或者拷贝成JNI类型并返回

JNI环境提供了大量的这些转换工作的工具可以帮助大家完成

在Java和Nativie之间传递参数

基础类型

    // In "win\jni_mh.h" - machine header which is machine dependent
  typedef 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;

String

JNI环境提供了函数可以进行很方便的转换

jstring -> char* : const char* GetStringUTFChars(JNIEnv*, jstring, jboolean*).
char* -> jstring : jstring NewStringUTF(JNIEnv*, char*).

例如：

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);
}

primitive数组

jintArray -> jint[], jint* GetIntArrayElements(JNIEnv *env, jintArray a, jboolean *iscopy).
jint[] -> jintArray, 首先分配内存jintArray NewIntArray(JNIEnv *env, jsize len), 然后填充void SetIntArrayRegion(JNIEnv *env, jintArray a, jsize start, jsize len, const jint *buf)

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;
}

访问对象变量并且调用Java方法

访问实例对象

GetObjectID
GetFieldID
GetInt
SetFieldID

JNIEXPORT void JNICALL Java_TestJNIInstanceVariable_modifyInstanceVariable
          (JNIEnv *env, jobject thisObj) {
   // Get a reference to this object's class
   jclass thisClass = (*env)->GetObjectClass(env, thisObj);

   // int
   // Get the Field ID of the instance variables "number"
   jfieldID fidNumber = (*env)->GetFieldID(env, thisClass, "number", "I");
   if (NULL == fidNumber) return;

   // Get the int given the Field ID
   jint number = (*env)->GetIntField(env, thisObj, fidNumber);
   printf("In C, the int is %d\n", number);

   // Change the variable
   number = 99;
   (*env)->SetIntField(env, thisObj, fidNumber, number);

   // Get the Field ID of the instance variables "message"
   jfieldID fidMessage = (*env)->GetFieldID(env, thisClass, "message", "Ljava/lang/String;");
   if (NULL == fidMessage) return;

   // String
   // Get the object given the Field ID
   jstring message = (*env)->GetObjectField(env, thisObj, fidMessage);

   // Create a C-string with the JNI String
   const char *cStr = (*env)->GetStringUTFChars(env, message, NULL);
   if (NULL == cStr) return;

   printf("In C, the string is %s\n", cStr);
   (*env)->ReleaseStringUTFChars(env, message, cStr);

   // Create a new C-string and assign to the JNI string
   message = (*env)->NewStringUTF(env, "Hello from C");
   if (NULL == message) return;

   // modify the instance variables
   (*env)->SetObjectField(env, thisObj, fidMessage, message);
}

访问静态变量

GetStaticFieldID
GetStaticField
SetStaticField

JNIEXPORT void JNICALL Java_TestJNIStaticVariable_modifyStaticVariable
          (JNIEnv *env, jobject thisObj) {
   // Get a reference to this object's class
   jclass cls = (*env)->GetObjectClass(env, thisObj);

   // Read the int static variable and modify its value
   jfieldID fidNumber = (*env)->GetStaticFieldID(env, cls, "number", "D");
   if (NULL == fidNumber) return;
   jdouble number = (*env)->GetStaticDoubleField(env, cls, fidNumber);
   printf("In C, the double is %f\n", number);
   number = 77.88;
   (*env)->SetStaticDoubleField(env, cls, fidNumber, number);
}

访问方法和静态方法

GetMethodID
CallMethod
GetStaticMethodID
CallStaticMethod

JNIEXPORT void JNICALL Java_TestJNICallBackMethod_nativeMethod
          (JNIEnv *env, jobject thisObj) {

   // Get a class reference for this object
   jclass thisClass = (*env)->GetObjectClass(env, thisObj);

   // Get the Method ID for method "callback", which takes no arg and return void
   jmethodID midCallBack = (*env)->GetMethodID(env, thisClass, "callback", "()V");
   if (NULL == midCallBack) return;
   printf("In C, call back Java's callback()\n");
   // Call back the method (which returns void), baed on the Method ID
   (*env)->CallVoidMethod(env, thisObj, midCallBack);

   jmethodID midCallBackStr = (*env)->GetMethodID(env, thisClass,
                               "callback", "(Ljava/lang/String;)V");
   if (NULL == midCallBackStr) return;
   printf("In C, call back Java's called(String)\n");
   jstring message = (*env)->NewStringUTF(env, "Hello from C");
   (*env)->CallVoidMethod(env, thisObj, midCallBackStr, message);

   jmethodID midCallBackAverage = (*env)->GetMethodID(env, thisClass,
                                  "callbackAverage", "(II)D");
   if (NULL == midCallBackAverage) return;
   jdouble average = (*env)->CallDoubleMethod(env, thisObj, midCallBackAverage, 2, 3);
   printf("In C, the average is %f\n", average);

   jmethodID midCallBackStatic = (*env)->GetStaticMethodID(env, thisClass,
                                 "callbackStatic", "()Ljava/lang/String;");
   if (NULL == midCallBackStatic) return;
   jstring resultJNIStr = (*env)->CallStaticObjectMethod(env, thisClass, midCallBackStatic);
   const char *resultCStr = (*env)->GetStringUTFChars(env, resultJNIStr, NULL);
   if (NULL == resultCStr) return;
   printf("In C, the returned string is %s\n", resultCStr);
   (*env)->ReleaseStringUTFChars(env, resultJNIStr, resultCStr);
}

创建java对象和对象数组

创建java对象

FindClass
NewObject
AllocObject

JNIEXPORT jobject JNICALL Java_TestJNIConstructor_getIntegerObject
          (JNIEnv *env, jobject thisObj, jint number) {
   // Get a class reference for java.lang.Integer
   jclass cls = (*env)->FindClass(env, "java/lang/Integer");

   // Get the Method ID of the constructor which takes an int
   jmethodID midInit = (*env)->GetMethodID(env, cls, "<init>", "(I)V");
   if (NULL == midInit) return NULL;
   // Call back constructor to allocate a new instance, with an int argument
   jobject newObj = (*env)->NewObject(env, cls, midInit, number);

   // Try runnning the toString() on this newly create object
   jmethodID midToString = (*env)->GetMethodID(env, cls, "toString", "()Ljava/lang/String;");
   if (NULL == midToString) return NULL;
   jstring resultStr = (*env)->CallObjectMethod(env, newObj, midToString);
   const char *resultCStr = (*env)->GetStringUTFChars(env, resultStr, NULL);
   printf("In C: the number is %s\n", resultCStr);

   return newObj;
}

创建java数组

NewObjectArray
GetObjectArrayElement
setObjectArrayElement

JNIEXPORT jobjectArray JNICALL Java_TestJNIObjectArray_sumAndAverage
          (JNIEnv *env, jobject thisObj, jobjectArray inJNIArray) {
   // Get a class reference for java.lang.Integer
   jclass classInteger = (*env)->FindClass(env, "java/lang/Integer");
   // Use Integer.intValue() to retrieve the int
   jmethodID midIntValue = (*env)->GetMethodID(env, classInteger, "intValue", "()I");
   if (NULL == midIntValue) return NULL;

   // Get the value of each Integer object in the array
   jsize length = (*env)->GetArrayLength(env, inJNIArray);
   jint sum = 0;
   int i;
   for (i = 0; i < length; i++) {
      jobject objInteger = (*env)->GetObjectArrayElement(env, inJNIArray, i);
      if (NULL == objInteger) return NULL;
      jint value = (*env)->CallIntMethod(env, objInteger, midIntValue);
      sum += value;
   }
   double average = (double)sum / length;
   printf("In C, the sum is %d\n", sum);
   printf("In C, the average is %f\n", average);

   // Get a class reference for java.lang.Double
   jclass classDouble = (*env)->FindClass(env, "java/lang/Double");

   // Allocate a jobjectArray of 2 java.lang.Double
   jobjectArray outJNIArray = (*env)->NewObjectArray(env, 2, classDouble, NULL);

   // Construct 2 Double objects by calling the constructor
   jmethodID midDoubleInit = (*env)->GetMethodID(env, classDouble, "<init>", "(D)V");
   if (NULL == midDoubleInit) return NULL;
   jobject objSum = (*env)->NewObject(env, classDouble, midDoubleInit, (double)sum);
   jobject objAve = (*env)->NewObject(env, classDouble, midDoubleInit, average);
   // Set to the jobjectArray
   (*env)->SetObjectArrayElement(env, outJNIArray, 0, objSum);
   (*env)->SetObjectArrayElement(env, outJNIArray, 1, objAve);

   return outJNIArray;
}

本地和全局变量

任何类似FindClass(), GetMethodID(), GetFieldID()返回的引用是一个本地引用。

想要使用全局引用的话，需要使用NewGlobalRef()，DeleteGlobalRef()

// Global Reference to the Java class "java.lang.Integer"
static jclass classInteger;
static jmethodID midIntegerInit;

jobject getInteger(JNIEnv *env, jobject thisObj, jint number) {

   // Get a class reference for java.lang.Integer if missing
   if (NULL == classInteger) {
      printf("Find java.lang.Integer\n");
    jclass classIntegerLocal = (*env)->FindClass(env, "java/lang/Integer");
      // Create a global reference from the local reference
      classInteger = (*env)->NewGlobalRef(env, classIntegerLocal);
      // No longer need the local reference, free it!
      //(*env)->DeleteLocalRef(env, classIntegerLocal);
   }

   if (NULL == classInteger) return NULL;

   // Get the Method ID of the Integer's constructor if missing
   if (NULL == midIntegerInit) {
      printf("Get Method ID for java.lang.Integer's constructor\n");
      midIntegerInit = (*env)->GetMethodID(env, classInteger, "<init>", "(I)V");
   }
   if (NULL == midIntegerInit) return NULL;

   // Call back constructor to allocate a new instance, with an int argument
   jobject newObj = (*env)->NewObject(env, classInteger, midIntegerInit, number);
   printf("In C, constructed java.lang.Integer with number %d\n", number);
   return newObj;
}

JNIEXPORT jobject JNICALL Java_TestJNIReference_getIntegerObject
          (JNIEnv *env, jobject thisObj, jint number) {
   return getInteger(env, thisObj, number);
}

JNIEXPORT jobject JNICALL Java_TestJNIReference_anotherGetIntegerObject
          (JNIEnv *env, jobject thisObj, jint number) {
   return getInteger(env, thisObj, number);
}