jni常见的使用

1. 访问java属性。

1. native接口
public class JniNatice {
    public String key = "andnux";
    static {
        System.loadLibrary("native");
    }
    public native String accessField();
}
2. c++实现
extern "C" JNIEXPORT jstring JNICALL
Java_top_andnux_jnidemo_JniNatice_accessField(JNIEnv *env, jobject instance) {
    jclass jclazz = env->GetObjectClass(instance);
    jfieldID fieldID = env->GetFieldID(jclazz, "key", "Ljava/lang/String;");
    jstring jstring1 = static_cast(env->GetObjectField(instance, fieldID));
    char *cstr = const_cast(env->GetStringUTFChars(jstring1, JNI_FALSE));
    char text[30] = "key ";
    strcat(text,cstr);
    jstring  jstring2 = env->NewStringUTF(text);
    env->SetObjectField(instance,fieldID,jstring2);
    return jstring2;
}

2. 访问java静态属性。

1. native接口
public class JniNatice {
    public static String key2 = "andnux";
    static {
        System.loadLibrary("native");
    }
    public native String accessStaticField();
}
2. c++实现
extern "C"
JNIEXPORT jstring JNICALL
Java_top_andnux_jnidemo_JniNatice_accessStaticField(JNIEnv *env, jobject instance) {
    jclass jclazz = env->GetObjectClass(instance);
    jfieldID fieldID = env->GetStaticFieldID(jclazz, "key2", "Ljava/lang/String;");
    jstring jstring1 = static_cast(env->GetStaticObjectField(jclazz, fieldID));
    char *cstr = const_cast(env->GetStringUTFChars(jstring1, JNI_FALSE));
    char text[30] = "key2 ";
    strcat(text,cstr);
    jstring  jstring2 = env->NewStringUTF(text);
    env->SetStaticObjectField(jclazz,fieldID,jstring2);
    return jstring2;
}

3. 访问java方法。

1. native接口
public class JniNatice {
    static {
        System.loadLibrary("native");
    }
    public int randomInt() {
        return new Random().nextInt();
    }
    public native int accessMethod();
}
2. c++实现
extern "C"
JNIEXPORT jint JNICALL
Java_top_andnux_jnidemo_JniNatice_accessMethod(JNIEnv *env, jobject instance) {
    jclass jclazz = env->GetObjectClass(instance);
    jmethodID jmethodID1 = env->GetMethodID(jclazz, "randomInt", "()I");
    jint value = env->CallIntMethod(instance, jmethodID1);
    return value;
}

4. 访问java静态方法。

1. native接口
public class JniNatice {
    static {
        System.loadLibrary("native");
    }
    public static String randomUUID() {
        return UUID.randomUUID().toString().replace("-", "");
    }
    public native int accessStaticMethod();
}
2. c++实现
extern "C"
JNIEXPORT jint JNICALL
Java_top_andnux_jnidemo_JniNatice_accessStaticMethod(JNIEnv *env, jobject instance) {
    jclass jclazz = env->GetObjectClass(instance);
    jmethodID jmethodID1 = env->GetStaticMethodID(jclazz, "randomUUID", "()Ljava/lang/String;");
    jstring jstring1 = static_cast(env->CallStaticObjectMethod(jclazz, jmethodID1));
    return jstring1;
}

5. 访问java构造方法。

1. native接口
public class JniNatice {
    static {
        System.loadLibrary("native");
    }
    public native long accessConstructor();
}
2. c++实现
extern "C"
JNIEXPORT jlong JNICALL
Java_top_andnux_jnidemo_JniNatice_accessConstructor(JNIEnv *env, jobject instance) {
    jclass jclazz = env->FindClass("java/util/Date");
    jmethodID jmethodID1 = env->GetMethodID(jclazz, "", "()V");
    jobject jobject1 = env->NewObject(jclazz, jmethodID1);
    jmethodID jmethodID2 = env->GetMethodID(jclazz, "getTime", "()J");
    jlong  jlong1 = env->CallLongMethod(jobject1,jmethodID2);
    return jlong1;
}

6. 访问java父类得方法。

1. A类
public class A {
    public void a() {
        Log.e("A", "aaaaaaaaa");
    }
}
2.AA类
public class AA extends A {
    @Override
    public void a() {
        Log.e("A", "AAAAAAAAAA");
    }
}
3. native接口
public class JniNatice {
    static {
        System.loadLibrary("native");
    }
     public native void accessNonvirtualMethod();
}
4. c++实现
extern "C"
JNIEXPORT void JNICALL
Java_top_andnux_jnidemo_JniNatice_accessNonvirtualMethod(JNIEnv *env, jobject instance) {
    jclass jclazz = env->GetObjectClass(instance);
    jfieldID jfieldID1 = env->GetFieldID(jclazz, "a", "Ltop/andnux/jnidemo/A;");
    jobject jobject1 = env->GetObjectField(instance, jfieldID1);
    jclass  jclass1 = env->FindClass("top/andnux/jnidemo/A");
    jmethodID  jmethodID1 = env->GetMethodID(jclass1,"a","()V");
    env->CallNonvirtualVoidMethod(jobject1,jclass1,jmethodID1);
}

7. 解决字符串乱码问题。

1. native接口
public class JniNatice {
    static {
        System.loadLibrary("native");
    }
    public native long accessConstructor();
}
2. c++实现
extern "C"
JNIEXPORT jstring JNICALL
Java_top_andnux_jnidemo_JniNatice_chineseChars(JNIEnv *env, jobject instance) {
    char *c_str = const_cast("张春林");
    jclass jclass1 = env->FindClass("java/lang/String");
    jmethodID jmethodID1 = env->GetMethodID(jclass1, "", "([BLjava/lang/String;)V");
    jbyteArray jbyteArray1 = env->NewByteArray(static_cast(strlen(c_str)));
    env->SetByteArrayRegion(jbyteArray1, 0, static_cast(strlen(c_str)),
                            reinterpret_cast(c_str));
    jstring charsetName = env->NewStringUTF("UTF-8");
    return static_cast(env->NewObject(jclass1, jmethodID1,
                                               jbyteArray1, charsetName));
//    return env->NewStringUTF(c_str);
}

8. 访问JAVA数组。

1. native接口
public class JniNatice {
    static {
        System.loadLibrary("native");
    }
    public native void  qsort(int [] datas);
}
2. c++实现
extern "C"
JNIEXPORT void JNICALL
Java_top_andnux_jnidemo_JniNatice_qsort(JNIEnv *env, jobject instance, jintArray datas_) {
    jint *datas = env->GetIntArrayElements(datas_, NULL);
    jint len =(env->GetArrayLength(datas_));
    qsort(datas, static_cast(len), sizeof(int),
          reinterpret_cast(comparator));
    env->ReleaseIntArrayElements(datas_, datas, 0);
}

9. 返回JAVA数组。

1. native接口
public class JniNatice {
    static {
        System.loadLibrary("native");
    }
    public native int[] getArray();
}
2. c++实现
extern "C"
JNIEXPORT jintArray JNICALL
Java_top_andnux_jnidemo_JniNatice_getArray(JNIEnv *env, jobject instance) {
    jintArray array = env->NewIntArray(10);
    jint len = env->GetArrayLength(array);
    jint *elems = env->GetIntArrayElements(array, JNI_FALSE);
    for (int i = 0; i < len; ++i) {
        elems[i] = i;
    }
    env->ReleaseIntArrayElements(array, elems, 0);
    return array;
}

10. 局部引用。

1. native接口
public class JniNatice {
    static {
        System.loadLibrary("native");
    }
    public native void localReference();
}
2. c++实现
extern "C"
JNIEXPORT void JNICALL
Java_top_andnux_jnidemo_JniNatice_localReference(JNIEnv *env, jobject instance) {
    for (int i = 0; i < 100; ++i) {
        jclass jclass1 = env->FindClass("java/util/Date");
        jmethodID jmethodID1 = env->GetMethodID(jclass1, "", "()V");
        jobject jobject1 = env->NewObject(jclass1, jmethodID1);
        env->DeleteLocalRef(jobject1);
    }
}

11. 全局引用(弱全局引用类似)。

弱全局引用：
节省内存，在内存不足的时候回收。
可以引用一个不常用的对象，如果为空，临时创建。
创建：NewWeakGlobalRef
销毁：DeleteWeakGlobalRef

1. native接口
public class JniNatice {
    static {
        System.loadLibrary("native");
    }

    public native void createGlobalReference();

    public native String getGlobalReference();

    public native void releaseGlobalReference();
}
2. c++实现
static jstring globalString;
extern "C"
JNIEXPORT void JNICALL
Java_top_andnux_jnidemo_JniNatice_createGlobalReference(JNIEnv *env, jobject instance) {
    jstring glob = env->NewStringUTF("哈哈哈哈");
    globalString = static_cast(env->NewGlobalRef(glob));
}

extern "C"
JNIEXPORT jstring JNICALL
Java_top_andnux_jnidemo_JniNatice_getGlobalReference(JNIEnv *env, jobject instance) {
    return globalString;
}

extern "C"
JNIEXPORT void JNICALL
Java_top_andnux_jnidemo_JniNatice_releaseGlobalReference(JNIEnv *env, jobject instance) {
    env->DeleteGlobalRef(globalString);
}

12. 异常处理。

1. native接口
public class JniNatice {
    static {
        System.loadLibrary("native");
    }
    public native String exception();
}
2. c++实现
extern "C"
JNIEXPORT jstring JNICALL
Java_top_andnux_jnidemo_JniNatice_exception(JNIEnv *env, jobject instance) {
    jclass jclazz = env->GetObjectClass(instance);
    jfieldID fieldID = env->GetFieldID(jclazz, "key2", "Ljava/lang/String;");
    jthrowable jthrowable1 = env->ExceptionOccurred();
    if (jthrowable1 != NULL) {
        env->ExceptionClear();
        fieldID = env->GetFieldID(jclazz, "key", "Ljava/lang/String;");
    }
    jstring jstring1 = static_cast(env->GetObjectField(instance, fieldID));
    char *ptr = (char *) env->GetStringChars(jstring1, JNI_FALSE);
    if (strcmp(ptr, "andnux") != 0) {
        jclass jclass1 = env->FindClass("java/lang/IllegalArgumentException");
        env->ThrowNew(jclass1, "参数错误");
    }
    return jstring1;
}

13. 附上签名。

基本类型	属性签名	方法签名
void	V	()
boolean	Z	(Z)V
byte	B	(B)V
char	C	(C)V
short	S	(S)V
int	I	(I)V
long	J	(J)V
float	F	(F)V
double	D	(D)V

引用类型的描述符:
一般引用类型则为 L + 该类型类描述符 + ;
(注意，这儿的分号“；”只得是JNI的一部分，而不是我们汉语中的分段，下同)

int[ ]     其描述符为[I
float[ ]   其描述符为[F
String[ ]  其描述符为[Ljava/lang/String;
Object[ ]类型的域描述符为[Ljava/lang/Object;

代码链接:https://github.com/andnux/JniDemo