基本数据类型

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//jni.h
//如果没在jni_md.h中定义好的话
#ifndef JNI_TYPES_ALREADY_DEFINED_IN_JNI_MD_H
//就使用下面的定义

typedef unsigned char   jboolean;
typedef unsigned short  jchar;
typedef short           jshort;
typedef float           jfloat;
typedef double          jdouble;

typedef jint            jsize;

接下来，我们找到找到jnt_md.h中的数据定义。 jni_md.h 包含了针对 jbyte、jint 和 jlong 的机器相关的类型定义。

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//jni.md.h
#ifdef TARGET_ARCH_x86 // 我们的环境是这个
# include "jni_x86.h"
#endif
#ifdef TARGET_ARCH_sparc
# include "jni_sparc.h"
#endif
#ifdef TARGET_ARCH_zero
# include "jni_zero.h"
#endif
#ifdef TARGET_ARCH_arm
# include "jni_arm.h"
#endif
#ifdef TARGET_ARCH_ppc
# include "jni_ppc.h"
#endif

那我们继续查看jint_x86.h：

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//jni_x86.h
  typedef int jint;
#if defined(_LP64)  // 64位机器
  typedef long jlong;
#else
  typedef long long jlong;
#endif
typedef signed char jbyte;

引用数据类型

下面是一些引用数据类型，使用typedef定义别名。

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#ifdef __cplusplus

class _jobject {};
class _jclass : public _jobject {};
class _jthrowable : public _jobject {};
class _jstring : public _jobject {};
class _jarray : public _jobject {};
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 _jobjectArray : public _jarray {};

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

弱引用类型

参考链接：https://juejin.cn/post/6934309297815977992

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typedef jobject jweak;

方法签名时用到的公共体类型jvalue

jvalue是一个unio（联合）类型。在C语中为了节约内存，会用联合类型变量来存储声明在联合体中的随意类型数据 。在JNI中将基本数据类型与引用类型定义在一个联合类型中，表示用jvalue定义的变量，能够存储随意JNI类型的数据。原型以下：

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

jfieldID和jMethodID

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struct _jfieldID;
typedef struct _jfieldID *jfieldID;

struct _jmethodID;
typedef struct _jmethodID *jmethodID;

四种不同引用类型的定义

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/* Return values from jobjectRefType */
typedef enum _jobjectType {
     JNIInvalidRefType    = 0,
     JNILocalRefType      = 1,
     JNIGlobalRefType     = 2,
     JNIWeakGlobalRefType = 3
} jobjectRefType;

布尔值

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/*
 * jboolean constants
 */

#define JNI_FALSE 0
#define JNI_TRUE 1

JNI函数的返回值

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/*
 * possible return values for JNI functions.
 */

#define JNI_OK           0                 /* success */
#define JNI_ERR          (-1)              /* unknown error */
#define JNI_EDETACHED    (-2)              /* thread detached from the VM */
#define JNI_EVERSION     (-3)              /* JNI version error */
#define JNI_ENOMEM       (-4)              /* not enough memory */
#define JNI_EEXIST       (-5)              /* VM already created */
#define JNI_EINVAL       (-6)              /* invalid arguments */

其他常量

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/*
 * used in ReleaseScalarArrayElements
 */

#define JNI_COMMIT 1
#define JNI_ABORT 2

JNI Native Method

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typedef struct {
    char *name;
    char *signature;
    void *fnPtr;
} JNINativeMethod;

JNI_ENV变量

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/*
 * JNI Native Method Interface.
 */

struct JNINativeInterface_;

struct JNIEnv_;

#ifdef __cplusplus
typedef JNIEnv_ JNIEnv;

struct JNINativeInterface_{
	........
    //很多函数指针
    ........
}
    
struct JNIEnv_ {
    const struct JNINativeInterface_ *functions;
#ifdef __cplusplus

    //很多方法，都是类似的格式
    jint GetVersion() {
        return functions->GetVersion(this);
    }

JavaVM变量

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/*
 * JNI Invocation Interface.
 */

struct JNIInvokeInterface_;

struct JavaVM_;

#ifdef __cplusplus
typedef JavaVM_ JavaVM;

struct JNIInvokeInterface_ {
    void *reserved0;
    void *reserved1;
    void *reserved2;

    jint (JNICALL *DestroyJavaVM)(JavaVM *vm);

    jint (JNICALL *AttachCurrentThread)(JavaVM *vm, void **penv, void *args);

    jint (JNICALL *DetachCurrentThread)(JavaVM *vm);

    jint (JNICALL *GetEnv)(JavaVM *vm, void **penv, jint version);

    jint (JNICALL *AttachCurrentThreadAsDaemon)(JavaVM *vm, void **penv, void *args);
};

struct JavaVM_ {
    const struct JNIInvokeInterface_ *functions;
#ifdef __cplusplus

    jint DestroyJavaVM() {
        return functions->DestroyJavaVM(this);
    }
    jint AttachCurrentThread(void **penv, void *args) {
        return functions->AttachCurrentThread(this, penv, args);
    }
    jint DetachCurrentThread() {
        return functions->DetachCurrentThread(this);
    }

    jint GetEnv(void **penv, jint version) {
        return functions->GetEnv(this, penv, version);
    }
    jint AttachCurrentThreadAsDaemon(void **penv, void *args) {
        return functions->AttachCurrentThreadAsDaemon(this, penv, args);
    }
#endif
};

其余的

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#ifdef _JNI_IMPLEMENTATION_
#define _JNI_IMPORT_OR_EXPORT_ JNIEXPORT
#else
#define _JNI_IMPORT_OR_EXPORT_ JNIIMPORT
#endif
_JNI_IMPORT_OR_EXPORT_ jint JNICALL
JNI_GetDefaultJavaVMInitArgs(void *args);

_JNI_IMPORT_OR_EXPORT_ jint JNICALL
JNI_CreateJavaVM(JavaVM **pvm, void **penv, void *args);

_JNI_IMPORT_OR_EXPORT_ jint JNICALL
JNI_GetCreatedJavaVMs(JavaVM **, jsize, jsize *);

/* Defined by native libraries. */
JNIEXPORT jint JNICALL
JNI_OnLoad(JavaVM *vm, void *reserved);

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

#define JNI_VERSION_1_1 0x00010001
#define JNI_VERSION_1_2 0x00010002
#define JNI_VERSION_1_4 0x00010004
#define JNI_VERSION_1_6 0x00010006
#define JNI_VERSION_1_8 0x00010008

#ifdef __cplusplus
} /* extern "C" */
#endif /* __cplusplus */

#endif /* !_JAVASOFT_JNI_H_ */

参考

https://blog.csdn.net/qq_36653924/article/details/124470927
https://www.jianshu.com/p/27eaa6669009
https://www.cnblogs.com/cynchanpin/p/7136178.html