jni 简介

jni的全称就是Java Native Interface，顾名思义，就是Java和C/C++相互通信的接口；

jni开发的代码会被编译成so文件，然后在java中加载so文件；

so文件加载

java 加载so文件主要是两种方式

1. loadlibrary

   调用 System.loadLibrary(libname) 或 Runtime.getRuntime().loadLibrary(libname) 加载 so 文件，只需要指定 so 文件的名字，就会默认从系统的共享库目录里面去查找；

2. load

   调用 System.load(filename) 或 Runtime.getRuntime().load(filename)加载so库，需要指定完整的so文件路径；

jni.h

jni 开发需要引用 jni.h；jni.h中主要包含以下部分：

1. 基本数据类型

   typedef uint8_t         jboolean;       /* unsigned 8 bits */
   typedef int8_t          jbyte;          /* signed 8 bits */
   typedef uint16_t        jchar;          /* unsigned 16 bits */
   typedef int16_t         jshort;         /* signed 16 bits */
   typedef int32_t         jint;           /* signed 32 bits */
   typedef int64_t         jlong;          /* signed 64 bits */
   typedef float           jfloat;         /* 32-bit IEEE 754 */
   typedef double          jdouble;        /* 64-bit IEEE 754 */
   typedef unsigned char   jboolean;       /* unsigned 8 bits */
   typedef signed char     jbyte;          /* signed 8 bits */
   typedef unsigned short  jchar;          /* unsigned 16 bits */
   typedef short           jshort;         /* signed 16 bits */
   typedef int             jint;           /* signed 32 bits */
   typedef long long       jlong;          /* signed 64 bits */
   typedef float           jfloat;         /* 32-bit IEEE 754 */
   typedef double          jdouble;        /* 64-bit IEEE 754 */
   
   typedef jint            jsize;          /* "cardinal indices and sizes" */
   

2. 数组类型

   class _jobject {};
   class _jclass : public _jobject {};
   class _jstring : public _jobject {};
   class _jarray : public _jobject {};
   class _jobjectArray : public _jarray {};
   class _jbooleanArray : public _jarray {};
   class _jbyteArray : public _jarray {};
   class _jcharArray : public _jarray {};
   class _jshortArray : public _jarray {};
   class _jintArray : public _jarray {};
   class _jlongArray : public _jarray {};
   class _jfloatArray : public _jarray {};
   class _jdoubleArray : public _jarray {};
   class _jthrowable : public _jobject {};
   
   typedef void*           jobject;
   typedef jobject         jclass;
   typedef jobject         jstring;
   typedef jobject         jarray;
   typedef jarray          jobjectArray;
   typedef jarray          jbooleanArray;
   typedef jarray          jbyteArray;
   typedef jarray          jcharArray;
   typedef jarray          jshortArray;
   typedef jarray          jintArray;
   typedef jarray          jlongArray;
   typedef jarray          jfloatArray;
   typedef jarray          jdoubleArray;
   typedef jobject         jthrowable;
   typedef jobject         jweak;
   

3. 公共体类型

   typedef union jvalue {
       jboolean    z;
       jbyte       b;
       jchar       c;
       jshort      s;
       jint        i;
       jlong       j;c
       jfloat      f;
       jdouble     d;
       jobject     l;
   } jvalue;
   

4. 方法id

   struct _jfieldID;                       /* opaque structure */
   typedef struct _jfieldID* jfieldID;     /* field IDs */
   

5. 方法id

   struct _jmethodID;                      /* opaque structure */
   typedef struct _jmethodID* jmethodID;   /* method IDs */
   

6. 引用类型定义

   typedef enum jobjectRefType {
       JNIInvalidRefType = 0,
       JNILocalRefType = 1,
       JNIGlobalRefType = 2,
       JNIWeakGlobalRefType = 3
   } jobjectRefType;
   

7. 错误类型定义

   #define JNI_FALSE   0
   #define JNI_TRUE    1
   
   #define JNI_OK          (0)         /* no error */
   #define JNI_ERR         (-1)        /* generic error */
   #define JNI_EDETACHED   (-2)        /* thread detached from the VM */
   #define JNI_EVERSION    (-3)        /* JNI version error */
   
   #define JNI_COMMIT      1           /* copy content, do not free buffer */
   #define JNI_ABORT       2           /* free buffer w/o copying back */
   

8. JNINativeInterface

   struct JNINativeInterface;
   struct _JNIEnv;
   
   typedef _JNIEnv JNIEnv;
   typedef const struct JNINativeInterface* C_JNIEnv;
   typedef const struct JNINativeInterface* JNIEnv;
   

9. JNIInvokeInterface

   struct JNIInvokeInterface_;  
   struct JavaVM_;  
   #ifdef __cplusplus  
   typedef JavaVM_ JavaVM;  
   #else  
   typedef const struct JNIInvokeInterface_ *JavaVM;  
   

10. 装载与卸载方法

    JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM* vm, void* reserved);
    JNIEXPORT void JNICALL JNI_OnUnload(JavaVM* vm, void* reserved);