这是JNI实现源码分析系列文章中的一部分，本系列文章结合Dalvik源码来说明JNI实现上的细节，本系列包括：
JNI实现源码分析【一 前言】
JNI实现源码分析【二 数据结构】
JNI实现源码分析【三 间接引用表】
JNI实现源码分析【四 函数调用】
JNI实现源码分析【五 结束语】
正文
在展开深入讨论之前，先说一下Dalvik中和JNI相关的数据结构，是很有必要的。
在Object.h中定义了很多的数据结构：
0x01: 虚拟机中的对象
我们知道，Java是面向对象的，Java是运行在虚拟机里面的，即先通过编译成字节码（dalvik对应dex），虚拟机解析字节码，构造出逻辑上相同的对象。 及虚拟机中的对象。
a. struct Object
根对象，就像我们知道，所有的对象都继承Object一样
b. struct ClassObject
虚拟机层面的类对象
c. struct DataObject
携带了数据的对象
d. struct StringObject
字符串对象
e. struct ArrayObject
数组对象
以上这几个结构，就把Java里面的对象全部表述清楚了。
0x02: Dex相关的结构
下面的结构在解析Dex时会用到，同样在JNI的实现中，也会用到，所以也说明一下。
a. struct Field
代表了类中的一个属性，通常通过解析Dex中的field构造而来
b. struct StaticField
代表了类中的一个static属性，通常通过解析Dex中的field构造而来
c. struct InstField
代表了类中的一个非static属性，通常通过解析Dex中的field构造而来
d. struct Method
代表了类中的一个方法，通常通过解析Dex中的method构造而来
0x03: 实现JNI需要的数据结构
虚拟机在实现JNI时，需要一些特殊的结构。有管理虚拟机的，有JNI操作相关的。比如我们知道在JNI的API中就有两个重要的结构：JavaVM,JNIEnv
a. JavaVM
typedef const struct JNIInvokeInterface* JavaVM;
原来JavaVM只是一个JNIInvokeInterface类型的指针
b. JavaVMExt
在代码中，我们经常看到JavaVMExt指针和JavaVM指针的互转，所以顺带说一下，JavaVMExt可以说是JavaVM的扩展，实际上确实如此，可以看到它包含了更多的信息:
struct JavaVMExt {
const struct JNIInvokeInterface* funcTable; /* must be first */
const struct JNIInvokeInterface* baseFuncTable;
/* head of list of JNIEnvs associated with this VM */
JNIEnvExt* envList;
pthread_mutex_t envListLock;
};
它的第一个结构也是JNIInvokeInterface类型的指针，所以可以安全的和JavaVM互转（当然只是对这个指针的使用的情况下）
c. JNIInvokeInterface
struct JNIInvokeInterface {
void* reserved0;
void* reserved1;
void* reserved2;
jint (*DestroyJavaVM)(JavaVM*);
jint (*AttachCurrentThread)(JavaVM*, JNIEnv**, void*);
jint (*DetachCurrentThread)(JavaVM*);
jint (*GetEnv)(JavaVM*, void**, jint);
jint (*AttachCurrentThreadAsDaemon)(JavaVM*, JNIEnv**, void*);
};
JNIInvokeInterface可以理解为函数表，其中包含了若干操作虚拟机的函数指针。
d. JNIEnv
typedef const struct JNINativeInterface* JNIEnv;
JNIEnv是一个JNINativeInterface类型的指针
e. JNIEnvExt
同样的，存在一个JNIEnv的扩展JNIEnvExt:
struct JNIEnvExt {
const struct JNINativeInterface* funcTable; /* must be first */
const struct JNINativeInterface* baseFuncTable;
u4 envThreadId;
Thread* self;
/* if nonzero, we are in a "critical" JNI call */
int critical;
struct JNIEnvExt* prev;
struct JNIEnvExt* next;
};
f. JNINativeInterface
struct JNINativeInterface {
void* reserved0;
void* reserved1;
void* reserved2;
void* reserved3;
jint (*GetVersion)(JNIEnv *);
jclass (*DefineClass)(JNIEnv*, const char*, jobject, const jbyte*,
jsize);
jclass (*FindClass)(JNIEnv*, const char*);
jmethodID (*FromReflectedMethod)(JNIEnv*, jobject);
jfieldID (*FromReflectedField)(JNIEnv*, jobject);
/* spec doesn't show jboolean parameter */
jobject (*ToReflectedMethod)(JNIEnv*, jclass, jmethodID, jboolean);
jclass (*GetSuperclass)(JNIEnv*, jclass);
jboolean (*IsAssignableFrom)(JNIEnv*, jclass, jclass);
/* spec doesn't show jboolean parameter */
jobject (*ToReflectedField)(JNIEnv*, jclass, jfieldID, jboolean);
jint (*Throw)(JNIEnv*, jthrowable);
jint (*ThrowNew)(JNIEnv *, jclass, const char *);
jthrowable (*ExceptionOccurred)(JNIEnv*);
void (*ExceptionDescribe)(JNIEnv*);
void (*ExceptionClear)(JNIEnv*);
void (*FatalError)(JNIEnv*, const char*);
jint (*PushLocalFrame)(JNIEnv*, jint);
jobject (*PopLocalFrame)(JNIEnv*, jobject);
jobject (*NewGlobalRef)(JNIEnv*, jobject);
void (*DeleteGlobalRef)(JNIEnv*, jobject);
void (*DeleteLocalRef)(JNIEnv*, jobject);
jboolean (*IsSameObject)(JNIEnv*, jobject, jobject);
jobject (*NewLocalRef)(JNIEnv*, jobject);
jint (*EnsureLocalCapacity)(JNIEnv*, jint);
jobject (*AllocObject)(JNIEnv*, jclass);
jobject (*NewObject)(JNIEnv*, jclass, jmethodID, ...);
jobject (*NewObjectV)(JNIEnv*, jclass, jmethodID, va_list);
jobject (*NewObjectA)(JNIEnv*, jclass, jmethodID, jvalue*);
jclass (*GetObjectClass)(JNIEnv*, jobject);
jboolean (*IsInstanceOf)(JNIEnv*, jobject, jclass);
jmethodID (*GetMethodID)(JNIEnv*, jclass, const char*, const char*);
jobject (*CallObjectMethod)(JNIEnv*, jobject, jmethodID, ...);
jobject (*CallObjectMethodV)(JNIEnv*, jobject, jmethodID, va_list);
jobject (*CallObjectMethodA)(JNIEnv*, jobject, jmethodID, jvalue*);
jboolean (*CallBooleanMethod)(JNIEnv*, jobject, jmethodID, ...);
jboolean (*CallBooleanMethodV)(JNIEnv*, jobject, jmethodID, va_list);
jboolean (*CallBooleanMethodA)(JNIEnv*, jobject, jmethodID, jvalue*);
jbyte (*CallByteMethod)(JNIEnv*, jobject, jmethodID, ...);
jbyte (*CallByteMethodV)(JNIEnv*, jobject, jmethodID, va_list);
jbyte (*CallByteMethodA)(JNIEnv*, jobject, jmethodID, jvalue*);
jchar (*CallCharMethod)(JNIEnv*, jobject, jmethodID, ...);
jchar (*CallCharMethodV)(JNIEnv*, jobject, jmethodID, va_list);
jchar (*CallCharMethodA)(JNIEnv*, jobject, jmethodID, jvalue*);
jshort (*CallShortMethod)(JNIEnv*, jobject, jmethodID, ...);
jshort (*CallShortMethodV)(JNIEnv*, jobject, jmethodID, va_list);
jshort (*CallShortMethodA)(JNIEnv*, jobject, jmethodID, jvalue*);
jint (*CallIntMethod)(JNIEnv*, jobject, jmethodID, ...);
jint (*CallIntMethodV)(JNIEnv*, jobject, jmethodID, va_list);
jint (*CallIntMethodA)(JNIEnv*, jobject, jmethodID, jvalue*);
jlong (*CallLongMethod)(JNIEnv*, jobject, jmethodID, ...);
jlong (*CallLongMethodV)(JNIEnv*, jobject, jmethodID, va_list);
jlong (*CallLongMethodA)(JNIEnv*, jobject, jmethodID, jvalue*);
jfloat (*CallFloatMethod)(JNIEnv*, jobject, jmethodID, ...);
jfloat (*CallFloatMethodV)(JNIEnv*, jobject, jmethodID, va_list);
jfloat (*CallFloatMethodA)(JNIEnv*, jobject, jmethodID, jvalue*);
jdouble (*CallDoubleMethod)(JNIEnv*, jobject, jmethodID, ...);
jdouble (*CallDoubleMethodV)(JNIEnv*, jobject, jmethodID, va_list);
jdouble (*CallDoubleMethodA)(JNIEnv*, jobject, jmethodID, jvalue*);
void (*CallVoidMethod)(JNIEnv*, jobject, jmethodID, ...);
void (*CallVoidMethodV)(JNIEnv*, jobject, jmethodID, va_list);
void (*CallVoidMethodA)(JNIEnv*, jobject, jmethodID, jvalue*);
jobject (*CallNonvirtualObjectMethod)(JNIEnv*, jobject, jclass,
jmethodID, ...);
jobject (*CallNonvirtualObjectMethodV)(JNIEnv*, jobject, jclass,
jmethodID, va_list);
jobject (*CallNonvirtualObjectMethodA)(JNIEnv*, jobject, jclass,
jmethodID, jvalue*);
jboolean (*CallNonvirtualBooleanMethod)(JNIEnv*, jobject, jclass,
jmethodID, ...);
jboolean (*CallNonvirtualBooleanMethodV)(JNIEnv*, jobject, jclass,
jmethodID, va_list);
jboolean (*CallNonvirtualBooleanMethodA)(JNIEnv*, jobject, jclass,
jmethodID, jvalue*);
jbyte (*CallNonvirtualByteMethod)(JNIEnv*, jobject, jclass,
jmethodID, ...);
jbyte (*CallNonvirtualByteMethodV)(JNIEnv*, jobject, jclass,
jmethodID, va_list);
jbyte (*CallNonvirtualByteMethodA)(JNIEnv*, jobject, jclass,
jmethodID, jvalue*);
jchar (*CallNonvirtualCharMethod)(JNIEnv*, jobject, jclass,
jmethodID, ...);
jchar (*CallNonvirtualCharMethodV)(JNIEnv*, jobject, jclass,
jmethodID, va_list);
jchar (*CallNonvirtualCharMethodA)(JNIEnv*, jobject, jclass,
jmethodID, jvalue*);
jshort (*CallNonvirtualShortMethod)(JNIEnv*, jobject, jclass,
jmethodID, ...);
jshort (*CallNonvirtualShortMethodV)(JNIEnv*, jobject, jclass,
jmethodID, va_list);
jshort (*CallNonvirtualShortMethodA)(JNIEnv*, jobject, jclass,
jmethodID, jvalue*);
jint (*CallNonvirtualIntMethod)(JNIEnv*, jobject, jclass,
jmethodID, ...);
jint (*CallNonvirtualIntMethodV)(JNIEnv*, jobject, jclass,
jmethodID, va_list);
jint (*CallNonvirtualIntMethodA)(JNIEnv*, jobject, jclass,
jmethodID, jvalue*);
jlong (*CallNonvirtualLongMethod)(JNIEnv*, jobject, jclass,
jmethodID, ...);
jlong (*CallNonvirtualLongMethodV)(JNIEnv*, jobject, jclass,
jmethodID, va_list);
jlong (*CallNonvirtualLongMethodA)(JNIEnv*, jobject, jclass,
jmethodID, jvalue*);
jfloat (*CallNonvirtualFloatMethod)(JNIEnv*, jobject, jclass,
jmethodID, ...);
jfloat (*CallNonvirtualFloatMethodV)(JNIEnv*, jobject, jclass,
jmethodID, va_list);
jfloat (*CallNonvirtualFloatMethodA)(JNIEnv*, jobject, jclass,
jmethodID, jvalue*);
jdouble (*CallNonvirtualDoubleMethod)(JNIEnv*, jobject, jclass,
jmethodID, ...);
jdouble (*CallNonvirtualDoubleMethodV)(JNIEnv*, jobject, jclass,
jmethodID, va_list);
jdouble (*CallNonvirtualDoubleMethodA)(JNIEnv*, jobject, jclass,
jmethodID, jvalue*);
void (*CallNonvirtualVoidMethod)(JNIEnv*, jobject, jclass,
jmethodID, ...);
void (*CallNonvirtualVoidMethodV)(JNIEnv*, jobject, jclass,
jmethodID, va_list);
void (*CallNonvirtualVoidMethodA)(JNIEnv*, jobject, jclass,
jmethodID, jvalue*);
jfieldID (*GetFieldID)(JNIEnv*, jclass, const char*, const char*);
jobject (*GetObjectField)(JNIEnv*, jobject, jfieldID);
jboolean (*GetBooleanField)(JNIEnv*, jobject, jfieldID);
jbyte (*GetByteField)(JNIEnv*, jobject, jfieldID);
jchar (*GetCharField)(JNIEnv*, jobject, jfieldID);
jshort (*GetShortField)(JNIEnv*, jobject, jfieldID);
jint (*GetIntField)(JNIEnv*, jobject, jfieldID);
jlong (*GetLongField)(JNIEnv*, jobject, jfieldID);
jfloat (*GetFloatField)(JNIEnv*, jobject, jfieldID);
jdouble (*GetDoubleField)(JNIEnv*, jobject, jfieldID);
void (*SetObjectField)(JNIEnv*, jobject, jfieldID, jobject);
void (*SetBooleanField)(JNIEnv*, jobject, jfieldID, jboolean);
void (*SetByteField)(JNIEnv*, jobject, jfieldID, jbyte);
void (*SetCharField)(JNIEnv*, jobject, jfieldID, jchar);
void (*SetShortField)(JNIEnv*, jobject, jfieldID, jshort);
void (*SetIntField)(JNIEnv*, jobject, jfieldID, jint);
void (*SetLongField)(JNIEnv*, jobject, jfieldID, jlong);
void (*SetFloatField)(JNIEnv*, jobject, jfieldID, jfloat);
void (*SetDoubleField)(JNIEnv*, jobject, jfieldID, jdouble);
jmethodID (*GetStaticMethodID)(JNIEnv*, jclass, const char*, const char*);
jobject (*CallStaticObjectMethod)(JNIEnv*, jclass, jmethodID, ...);
jobject (*CallStaticObjectMethodV)(JNIEnv*, jclass, jmethodID, va_list);
jobject (*CallStaticObjectMethodA)(JNIEnv*, jclass, jmethodID, jvalue*);
jboolean (*CallStaticBooleanMethod)(JNIEnv*, jclass, jmethodID, ...);
jboolean (*CallStaticBooleanMethodV)(JNIEnv*, jclass, jmethodID,
va_list);
jboolean (*CallStaticBooleanMethodA)(JNIEnv*, jclass, jmethodID,
jvalue*);
jbyte (*CallStaticByteMethod)(JNIEnv*, jclass, jmethodID, ...);
jbyte (*CallStaticByteMethodV)(JNIEnv*, jclass, jmethodID, va_list);
jbyte (*CallStaticByteMethodA)(JNIEnv*, jclass, jmethodID, jvalue*);
jchar (*CallStaticCharMethod)(JNIEnv*, jclass, jmethodID, ...);
jchar (*CallStaticCharMethodV)(JNIEnv*, jclass, jmethodID, va_list);
jchar (*CallStaticCharMethodA)(JNIEnv*, jclass, jmethodID, jvalue*);
jshort (*CallStaticShortMethod)(JNIEnv*, jclass, jmethodID, ...);
jshort (*CallStaticShortMethodV)(JNIEnv*, jclass, jmethodID, va_list);
jshort (*CallStaticShortMethodA)(JNIEnv*, jclass, jmethodID, jvalue*);
jint (*CallStaticIntMethod)(JNIEnv*, jclass, jmethodID, ...);
jint (*CallStaticIntMethodV)(JNIEnv*, jclass, jmethodID, va_list);
jint (*CallStaticIntMethodA)(JNIEnv*, jclass, jmethodID, jvalue*);
jlong (*CallStaticLongMethod)(JNIEnv*, jclass, jmethodID, ...);
jlong (*CallStaticLongMethodV)(JNIEnv*, jclass, jmethodID, va_list);
jlong (*CallStaticLongMethodA)(JNIEnv*, jclass, jmethodID, jvalue*);
jfloat (*CallStaticFloatMethod)(JNIEnv*, jclass, jmethodID, ...);
jfloat (*CallStaticFloatMethodV)(JNIEnv*, jclass, jmethodID, va_list);
jfloat (*CallStaticFloatMethodA)(JNIEnv*, jclass, jmethodID, jvalue*);
jdouble (*CallStaticDoubleMethod)(JNIEnv*, jclass, jmethodID, ...);
jdouble (*CallStaticDoubleMethodV)(JNIEnv*, jclass, jmethodID, va_list);
jdouble (*CallStaticDoubleMethodA)(JNIEnv*, jclass, jmethodID, jvalue*);
void (*CallStaticVoidMethod)(JNIEnv*, jclass, jmethodID, ...);
void (*CallStaticVoidMethodV)(JNIEnv*, jclass, jmethodID, va_list);
void (*CallStaticVoidMethodA)(JNIEnv*, jclass, jmethodID, jvalue*);
jfieldID (*GetStaticFieldID)(JNIEnv*, jclass, const char*,
const char*);
jobject (*GetStaticObjectField)(JNIEnv*, jclass, jfieldID);
jboolean (*GetStaticBooleanField)(JNIEnv*, jclass, jfieldID);
jbyte (*GetStaticByteField)(JNIEnv*, jclass, jfieldID);
jchar (*GetStaticCharField)(JNIEnv*, jclass, jfieldID);
jshort (*GetStaticShortField)(JNIEnv*, jclass, jfieldID);
jint (*GetStaticIntField)(JNIEnv*, jclass, jfieldID);
jlong (*GetStaticLongField)(JNIEnv*, jclass, jfieldID);
jfloat (*GetStaticFloatField)(JNIEnv*, jclass, jfieldID);
jdouble (*GetStaticDoubleField)(JNIEnv*, jclass, jfieldID);
void (*SetStaticObjectField)(JNIEnv*, jclass, jfieldID, jobject);
void (*SetStaticBooleanField)(JNIEnv*, jclass, jfieldID, jboolean);
void (*SetStaticByteField)(JNIEnv*, jclass, jfieldID, jbyte);
void (*SetStaticCharField)(JNIEnv*, jclass, jfieldID, jchar);
void (*SetStaticShortField)(JNIEnv*, jclass, jfieldID, jshort);
void (*SetStaticIntField)(JNIEnv*, jclass, jfieldID, jint);
void (*SetStaticLongField)(JNIEnv*, jclass, jfieldID, jlong);
void (*SetStaticFloatField)(JNIEnv*, jclass, jfieldID, jfloat);
void (*SetStaticDoubleField)(JNIEnv*, jclass, jfieldID, jdouble);
jstring (*NewString)(JNIEnv*, const jchar*, jsize);
jsize (*GetStringLength)(JNIEnv*, jstring);
const jchar* (*GetStringChars)(JNIEnv*, jstring, jboolean*);
void (*ReleaseStringChars)(JNIEnv*, jstring, const jchar*);
jstring (*NewStringUTF)(JNIEnv*, const char*);
jsize (*GetStringUTFLength)(JNIEnv*, jstring);
/* JNI spec says this returns const jbyte*, but that's inconsistent */
const char* (*GetStringUTFChars)(JNIEnv*, jstring, jboolean*);
void (*ReleaseStringUTFChars)(JNIEnv*, jstring, const char*);
jsize (*GetArrayLength)(JNIEnv*, jarray);
jobjectArray (*NewObjectArray)(JNIEnv*, jsize, jclass, jobject);
jobject (*GetObjectArrayElement)(JNIEnv*, jobjectArray, jsize);
void (*SetObjectArrayElement)(JNIEnv*, jobjectArray, jsize, jobject);
jbooleanArray (*NewBooleanArray)(JNIEnv*, jsize);
jbyteArray (*NewByteArray)(JNIEnv*, jsize);
jcharArray (*NewCharArray)(JNIEnv*, jsize);
jshortArray (*NewShortArray)(JNIEnv*, jsize);
jintArray (*NewIntArray)(JNIEnv*, jsize);
jlongArray (*NewLongArray)(JNIEnv*, jsize);
jfloatArray (*NewFloatArray)(JNIEnv*, jsize);
jdoubleArray (*NewDoubleArray)(JNIEnv*, jsize);
jboolean* (*GetBooleanArrayElements)(JNIEnv*, jbooleanArray, jboolean*);
jbyte* (*GetByteArrayElements)(JNIEnv*, jbyteArray, jboolean*);
jchar* (*GetCharArrayElements)(JNIEnv*, jcharArray, jboolean*);
jshort* (*GetShortArrayElements)(JNIEnv*, jshortArray, jboolean*);
jint* (*GetIntArrayElements)(JNIEnv*, jintArray, jboolean*);
jlong* (*GetLongArrayElements)(JNIEnv*, jlongArray, jboolean*);
jfloat* (*GetFloatArrayElements)(JNIEnv*, jfloatArray, jboolean*);
jdouble* (*GetDoubleArrayElements)(JNIEnv*, jdoubleArray, jboolean*);
void (*ReleaseBooleanArrayElements)(JNIEnv*, jbooleanArray,
jboolean*, jint);
void (*ReleaseByteArrayElements)(JNIEnv*, jbyteArray,
jbyte*, jint);
void (*ReleaseCharArrayElements)(JNIEnv*, jcharArray,
jchar*, jint);
void (*ReleaseShortArrayElements)(JNIEnv*, jshortArray,
jshort*, jint);
void (*ReleaseIntArrayElements)(JNIEnv*, jintArray,
jint*, jint);
void (*ReleaseLongArrayElements)(JNIEnv*, jlongArray,
jlong*, jint);
void (*ReleaseFloatArrayElements)(JNIEnv*, jfloatArray,
jfloat*, jint);
void (*ReleaseDoubleArrayElements)(JNIEnv*, jdoubleArray,
jdouble*, jint);
void (*GetBooleanArrayRegion)(JNIEnv*, jbooleanArray,
jsize, jsize, jboolean*);
void (*GetByteArrayRegion)(JNIEnv*, jbyteArray,
jsize, jsize, jbyte*);
void (*GetCharArrayRegion)(JNIEnv*, jcharArray,
jsize, jsize, jchar*);
void (*GetShortArrayRegion)(JNIEnv*, jshortArray,
jsize, jsize, jshort*);
void (*GetIntArrayRegion)(JNIEnv*, jintArray,
jsize, jsize, jint*);
void (*GetLongArrayRegion)(JNIEnv*, jlongArray,
jsize, jsize, jlong*);
void (*GetFloatArrayRegion)(JNIEnv*, jfloatArray,
jsize, jsize, jfloat*);
void (*GetDoubleArrayRegion)(JNIEnv*, jdoubleArray,
jsize, jsize, jdouble*);
/* spec shows these without const; some jni.h do, some don't */
void (*SetBooleanArrayRegion)(JNIEnv*, jbooleanArray,
jsize, jsize, const jboolean*);
void (*SetByteArrayRegion)(JNIEnv*, jbyteArray,
jsize, jsize, const jbyte*);
void (*SetCharArrayRegion)(JNIEnv*, jcharArray,
jsize, jsize, const jchar*);
void (*SetShortArrayRegion)(JNIEnv*, jshortArray,
jsize, jsize, const jshort*);
void (*SetIntArrayRegion)(JNIEnv*, jintArray,
jsize, jsize, const jint*);
void (*SetLongArrayRegion)(JNIEnv*, jlongArray,
jsize, jsize, const jlong*);
void (*SetFloatArrayRegion)(JNIEnv*, jfloatArray,
jsize, jsize, const jfloat*);
void (*SetDoubleArrayRegion)(JNIEnv*, jdoubleArray,
jsize, jsize, const jdouble*);
jint (*RegisterNatives)(JNIEnv*, jclass, const JNINativeMethod*,
jint);
jint (*UnregisterNatives)(JNIEnv*, jclass);
jint (*MonitorEnter)(JNIEnv*, jobject);
jint (*MonitorExit)(JNIEnv*, jobject);
jint (*GetJavaVM)(JNIEnv*, JavaVM**);
void (*GetStringRegion)(JNIEnv*, jstring, jsize, jsize, jchar*);
void (*GetStringUTFRegion)(JNIEnv*, jstring, jsize, jsize, char*);
void* (*GetPrimitiveArrayCritical)(JNIEnv*, jarray, jboolean*);
void (*ReleasePrimitiveArrayCritical)(JNIEnv*, jarray, void*, jint);
const jchar* (*GetStringCritical)(JNIEnv*, jstring, jboolean*);
void (*ReleaseStringCritical)(JNIEnv*, jstring, const jchar*);
jweak (*NewWeakGlobalRef)(JNIEnv*, jobject);
void (*DeleteWeakGlobalRef)(JNIEnv*, jweak);
jboolean (*ExceptionCheck)(JNIEnv*);
jobject (*NewDirectByteBuffer)(JNIEnv*, void*, jlong);
void* (*GetDirectBufferAddress)(JNIEnv*, jobject);
jlong (*GetDirectBufferCapacity)(JNIEnv*, jobject);
/* added in JNI 1.6 */
jobjectRefType (*GetObjectRefType)(JNIEnv*, jobject);
};
JNINativeInterface可以理解为函数表，其中包含了JNI API提供的所有操作。
0x04: JNI参数传递的数据结构
方法调用少不了参数传递：入参和返回值，在JNI中，并不是直接将虚拟机中的Object暴露给API，而是使用了形如jobject这样的结构，为何？主要还是安全方面的考虑，假如暴露了Object，那就可以在JNI中随意调整Object的结构，这是框架所不愿意看到的。
一起看看JNI中的参数的数据结构：
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;
所以的参数看起来都是指针，但是需要注意，jobject并不是指向Object的指针，后面我们在讲IndirectRefTable的时候会解释。
而值得一提的是：
struct _jfieldID; /* opaque structure */
typedef struct _jfieldID* jfieldID; /* field IDs */
struct _jmethodID; /* opaque structure */
typedef struct _jmethodID* jmethodID; /* method IDs */
jfieldID和jmethodID 却是指向真实Field和Method的指针。
