JNI:jni.h 头文件

在jni.h头文件中,定义了引用类型。

//所有引用类型的基类_jobject 
class _jobject {};
//所有类类型
class _jclass : public _jobject {};
//异常类型
class _jthrowable : public _jobject {};
//字符串类型
class _jstring : public _jobject {};
//数组类型
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 {};
//对象数组类型
class _jobjectArray : public _jarray {};

在jni中值类是使用union来组织：

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

对于字段和属性的定义：

struct _jfieldID;
typedef struct _jfieldID *jfieldID;

struct _jmethodID;
typedef struct _jmethodID *jmethodID;

在jni中所有引用类型的类别分为：局部引用类型,全局引用类型和全局弱引用类型。

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

在jni中注册的方法约定如下：

/*
 * used in RegisterNatives to describe native method name, signature,
 * and function pointer.
 */

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

struct JNIInvokeInterface_; 结果类型为函数指针数组：

struct JNINativeInterface_ {
    void *reserved0;
    void *reserved1;
    void *reserved2;
    void *reserved3;
    jint (JNICALL *GetVersion)(JNIEnv *env);
    jclass (JNICALL *DefineClass)
      (JNIEnv *env, const char *name, jobject loader, const jbyte *buf,
       jsize len);
    jclass (JNICALL *FindClass)
      (JNIEnv *env, const char *name);
    /* New JNI 1.6 Features */
    jobjectRefType (JNICALL *GetObjectRefType)
        (JNIEnv* env, jobject obj);
}

JNIEnv_ 为包含JNINativeInterface_ 指针的指针。

/*
 * We use inlined functions for C++ so that programmers can write:
 *
 *    env->FindClass("java/lang/String")
 *
 * in C++ rather than:
 *
 *    (*env)->FindClass(env, "java/lang/String")
 *
 * in C.
 */

struct JNIEnv_ {
    const struct JNINativeInterface_ *functions;
    jclass DefineClass(const char *name, jobject loader, const jbyte *buf,
                       jsize len) {
        return functions->DefineClass(this, name, loader, buf, len);
    }
    jclass FindClass(const char *name) {
        return functions->FindClass(this, name);
    }
}

JavaVM的定义如下：

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;
    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);
    }
};

在让我们看看java中万物之尊,Java的Object对象的源代码。

Object.java源代码：

public class Object {

    private static native void registerNatives();
    static {
        registerNatives();
    }
	public final native Class<?> getClass();
	public native int hashCode();
}

在装载Object类时,调用本地方法registerNatives()方法。在Object中存在getClass() hashCode()等本地方法。
Object.c源代码：

static JNINativeMethod methods[] = {
    {"hashCode",    "()I",                    (void *)&JVM_IHashCode},
};
这里注册方法都必须尊重这个规则。在Lua API中也有类似的东西。
JNIEXPORT void JNICALL
Java_java_lang_Object_registerNatives(JNIEnv *env, jclass cls)
{
    (*env)->RegisterNatives(env, cls,
                            methods, sizeof(methods)/sizeof(methods[0]));
}

JNIEXPORT jclass JNICALL
Java_java_lang_Object_getClass(JNIEnv *env, jobject this)
{
    if (this == NULL) {
        JNU_ThrowNullPointerException(env, NULL);
        return 0;
    } else {
        return (*env)->GetObjectClass(env, this);
    }
}

由此看来,hashCode()源代码是由JVM来实现的。呜呜呜。好复杂的说。