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JNI SAMPLES

参考文献： https://www3.ntu.edu.sg/home/ehchua/programming/java/JavaNativeInterface.html

目录

  • 1_java_call_c (java调用c方法)
  • 2_java_call_c++ (java调用c++方法)
  • 3_passing_args_results (参数和结果的传递)
  • 4_modify_instance_variable (修改instance对象变量)
  • 5_callback_java (c回调java方法)
  • 6_c中创建java对象（doing...）
  • 7_c中创建全局引用缓存（doing...）

基本流程

  1. java类中声明native方法
  2. javac -h 生成class及对应.h头文件
  3. 编写对应的.c实现相关逻辑（通常为对已有c的库做java适配）
  4. 编译c，生成动态链接库
  5. java执行(引入动态链接库)

JNI常用方法

JNI String方法

const char * GetStringUTFChars(JNIEnv *env, jstring string, jboolean *isCopy);
   // 返回UTF8编码String的char*数组
void ReleaseStringUTFChars(JNIEnv *env, jstring string, const char *utf);
   // 通知VM native不再需要string了 
jstring NewStringUTF(JNIEnv *env, const char *bytes);
   // 返回一个新的UTF String对象
jsize GetStringUTFLength(JNIEnv *env, jstring string);
   // 返回UTF8 编码的String长度
void GetStringUTFRegion(JNIEnv *env, jstring str, jsize start, jsize length, char *buf);
   // 获取部分长度并copy到新数组
  
// UTF8 换成了 Unicode
const jchar * GetStringChars(JNIEnv *env, jstring string, jboolean *isCopy);
void ReleaseStringChars(JNIEnv *env, jstring string, const jchar *chars);
jstring NewString(JNIEnv *env, const jchar *unicodeChars, jsize length);
jsize GetStringLength(JNIEnv *env, jstring string);
void GetStringRegion(JNIEnv *env, jstring str, jsize start, jsize length, jchar *buf);

JNI Primitive Array方法

// 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, jboolean
NativeType * 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);

JNI instance or static 参数访问

jclass GetObjectClass(JNIEnv *env, jobject obj);
   // 返回对象的class.
   
jfieldID GetFieldID(JNIEnv *env, jclass cls, const char *name, const char *sig);
   // 返回fieldId
 
NativeType Get<type>Field(JNIEnv *env, jobject obj, jfieldID fieldID);
void Set<type>Field(JNIEnv *env, jobject obj, jfieldID fieldID, NativeType value);
   // intance field值的 get set
   

// static Field
jfieldID GetStaticFieldID(JNIEnv *env, jclass cls, const char *name, const char *sig);
NativeType GetStatic<type>Field(JNIEnv *env, jclass clazz, jfieldID fieldID);
void SetStatic<type>Field(JNIEnv *env, jclass clazz, jfieldID fieldID, NativeType value);

c调用java instance方法

jmethodID GetMethodID(JNIEnv *env, jclass cls, const char *name, const char *sig);
   // 返回instance/class 的MethodId
NativeType Call<type>Method(JNIEnv *env, jobject obj, jmethodID methodID, ...);
NativeType Call<type>MethodA(JNIEnv *env, jobject obj, jmethodID methodID, const jvalue *args);
NativeType Call<type>MethodV(JNIEnv *env, jobject obj, jmethodID methodID, va_list args);
   // 调用Method方法
   // <type> includes each of the eight primitive and Object.
   
// static 方法
jmethodID GetStaticMethodID(JNIEnv *env, jclass cls, const char *name, const char *sig);
NativeType CallStatic<type>Method(JNIEnv *env, jclass clazz, jmethodID methodID, ...);
NativeType CallStatic<type>MethodA(JNIEnv *env, jclass clazz, jmethodID methodID, const jvalue *args);
NativeType CallStatic<type>MethodV(JNIEnv *env, jclass clazz, jmethodID methodID, va_list args);