使用jni直接访问java对象中的字段

Java类：UserInfo

public class UserInfo {
	static {
		System.loadLibrary("userinfo");
	}
	// 从JNI中直接访问字段
	public String mUserName;
	public int mUserAge;
	public long mMoney;
	
	public UserInfo() {
		
	}
	
	public int getUserAge() {
		return mUserAge;
	}
	
	public native void setUserName(String name);
	public native void setUserAge(int age);
	public native void setUserMoney(long money);
}

1、FindClass：找到某个类，并返回jclass对象

2、GetObjectClass: 获取到某个对象的类型，并返回jclass对象

3、GetFieldID：获取到字段的ID

4、SetIntField/SetLongField/SetObjectField：为字段设值

基于上面的几个方法，可以编写代码如下：

/*
 * Class:     com_iflytek_testjni_UserInfo
 * Method:    setUserName
 * Signature: (Ljava/lang/String;)V
 */
JNIEXPORT void JNICALL Java_com_iflytek_testjni_UserInfo_setUserName(JNIEnv *env, jobject obj, jstring jstr) {
	jfieldID fid;

	jclass cls = (*env)->GetObjectClass(env, obj);
	fid = (*env)->GetFieldID(env, cls, "mUserName", "Ljava/lang/String;");
	(*env)->SetObjectField(env, obj, fid, jstr);
}

/*
 * Class:     com_iflytek_testjni_UserInfo
 * Method:    setUserAge
 * Signature: (I)V
 */
JNIEXPORT void JNICALL Java_com_iflytek_testjni_UserInfo_setUserAge(JNIEnv *env, jobject obj, jint age) {
	jclass objClass = (*env)->FindClass(env, "com/iflytek/testjni/UserInfo");
	jfieldID ageID = (*env)->GetFieldID(env, objClass, "mUserAge", "I");
	(*env)->SetIntField(env, obj, ageID, (int)age);
}

/*
 * Class:     com_iflytek_testjni_UserInfo
 * Method:    setUserMoney
 * Signature: (J)V
 */
JNIEXPORT void JNICALL Java_com_iflytek_testjni_UserInfo_setUserMoney(JNIEnv *env, jobject obj, jlong money) {
	jclass objClass = (*env)->FindClass(env, "com/iflytek/testjni/UserInfo");
	jfieldID id = (*env)->GetFieldID(env, objClass, "mMoney", "J");
	(*env)->SetLongField(env, obj, id, (long)money);
}

注意：由于方法签名、字段名称都是写死在C语言中的，因此代码不能混淆。

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