java jni笔记

1. 在java中定义native method
• public native void set_intValue(int selfHashCode, int v);
2. 从java中生成C++用头文件
• javah -jni -classpath bin -d hdr HelloWorld
3. C++ project的配置
• include目录 c:\Program Files\Java\jdk1.7.0_02\include
• include目录 c:\Program Files\Java\jdk1.7.0_02\include\win32
• include目录 对应javaclass.h的位置
4. C++中基本类型
1. JNIENV - java的运行环境
2. jobject - 代表java的instance
3. jclass - 代表java的类
5. 基本使用
• 在java中load C++的native 库

  System.loadLibrary("myjnilib");

• C++ ：根据jobject获取其jclass

  jclass cls = (*env)->GetObjectClass(env, obj);

• C++ ， 获取methodId

      jmethodID mid = (*env)->GetMethodID(env, cls, "callback", "(I)V");
      if (mid == 0) {
          return;
      }

• 此处， 最后一个参数为函数签名， 格式为 ( arg-types ) ret-type， I 为int， V为void， 参见 http://www.iam.ubc.ca/guides/javatut99/native1.1/implementing/method.html
• 可以用javap来生成java的类的签名列表， 如javap -s -p bin\HelloWorld
• C++， 调用Java method

(*env)->CallVoidMethod(env, obj, mid, depth);

• 其他返回类型的调用参见JNIENV的定义
• C++， 访问java的String
JNIEXPORT jstring JNICALL
Java_Prompt_getLine(JNIEnv *env, jobject obj, jstring prompt)
{
    char buf[128];
    const char *str = (*env)->GetStringUTFChars(env, prompt, 0);
    printf("%s", str);
    (*env)->ReleaseStringUTFChars(env, prompt, str);

• C++， 由C++String 到 java String

 ...
    scanf("%s", buf);
    return (*env)->NewStringUTF(env, buf);
}

• 其他String相关的function
• GetStringChars takes the Java string and returns a pointer to an array of Unicode characters that comprise the string.
• ReleaseStringChars releases the pointer to the array of Unicode characters.
• NewString constructs a new java.lang.String object from an array of Unicode characters.
• GetStringLength returns the length of a string that is comprised of an array of Unicode characters.
• GetStringUTFLength returns the length of a string if it is represented in the UTF-8 format
• C++， 访问java Array

JNIEXPORT jint JNICALL
Java_IntArray_sumArray(JNIEnv *env, jobject obj, jintArray arr)
{
    int i, sum = 0;
    jsize len = (*env)->GetArrayLength(env, arr);
    ...

• 访问array的元素
• GetObjectArrayElement returns the object element at a given index.
• SetObjectArrayElement updates the object element at a given index

jint *body = (*env)->GetIntArrayElements(env, arr, 0);
    for (i=0; i<len; i++) {
        sum += body[i];
    }

• 释放

(*env)->ReleaseIntArrayElements(env, arr, body, 0);
    return sum;

• C++, 访问Java field

//get field id

fid = (*env)->GetStaticFieldID(env, cls, "si", "I");

fid = (*env)->GetFieldID(env, cls, "s", "Ljava/lang/String;");

//get field value

si = (*env)->GetStaticIntField(env, cls, fid);

jstr = (*env)->GetObjectField(env, obj, fid);

• C++, 处理java中的Exception

JNIEXPORT void JNICALL 
Java_CatchThrow_catchThrow(JNIEnv *env, jobject obj)
{
  jclass cls = (*env)->GetObjectClass(env, obj);
  jmethodID mid = (*env)->GetMethodID(env, cls, "callback", "()V");
  jthrowable exc;
  if (mid == 0) {
    return;
  }
  (*env)->CallVoidMethod(env, obj, mid);
  exc = (*env)->ExceptionOccurred(env);
  if (exc) {
    /* We don't do much with the exception, except that we print a
       debug message using ExceptionDescribe, clear it, and throw
       a new exception. */
    jclass newExcCls;

    (*env)->ExceptionDescribe(env);
    (*env)->ExceptionClear(env);

    newExcCls = (*env)->FindClass(env, "java/lang/IllegalArgumentException");
    if (newExcCls == 0) { /* Unable to find the new exception class, give up. */
      return;
    }
    (*env)->ThrowNew(env, newExcCls, "thrown from C code");
  }
}

• It is extremely important to check, handle, and clear the pending exception before calling subsequent JNI functions. Calling arbitrary JNI functions with a pending exception may lead to unexpected results. You can safely call only a small number of JNI functions when there is a pending exception. These functions are ExceptionOccurred, ExceptionDescribe, and ExceptionClear.
• 线程
• The JNI interface pointer (JNIEnv *) is only valid in the current thread.
• You must not pass local references from one thread to another. In particular, a local reference may become invalid before the other thread has had a chance to use it. You should always convert local references to global references in situations where different threads may be using the same reference to a Java object.
• Check the use of global variables carefully. Multiple threads might be accessing these global variables at the same time. Make sure you put in appropriate locks to ensure safety.
• Native code can perform equivalent synchronization on objects using the JNI functions MonitorEnter andMonitorExit. For example:

...
(*env)->MonitorEnter(env, obj);
...                      /* synchronized block */
(*env)->MonitorExit(env, obj);
...
• C++， 根据名字查找类

jclass cls = env->FindClass("java/lang/String");