JNI实现源码分析【二 数据结构】

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。

作者：difcareer
链接：http://www.jianshu.com/p/e0d0c8923ae5
來源：简书

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