如何通过 JNI 设置 Class 类型变量的 "Double" 值？

Question

如果我只想将值设置为 Double 类型变量，我可能会这样编码：

public static native int getDoubleVar(Double dobj);
JNIEXPORT jint JNICALL
test_jni_Native_testGet(JNIEnv *env, jclass type, jobject dobj)
{
    jclass DoubleClass = env->FindClass("java/lang/Double");
    jfieldID valueID = env->GetFieldID(DoubleClass, "value", "D");
    env->SetDoubleField(dobj, valueID, 2.3);
    return 0;
}

这些代码生效。

但是，当我试图通过 JNI 设置 class 的 "Double" 变量的值时，我无法得到我想要的，当然，它崩溃了，这让我有点困惑。

Java 代码：

public class TestDouble
{

    public Double value;
    public long num;
    public TestDouble(long num, Double value)
    {
        this.num = num;
        this.value = value;
    }
}

母语：

public static native int testGet(TestDouble tdobj);

c 代码：

JNIEXPORT jint JNICALL
test_jni_Native_testGet(JNIEnv *env, jclass type, jobject tdobj)
{
    jclass tdobjClass = env->FindClass("xxxx/TestDouble");
    jfieldID valueID = env->GetFieldID(tdobjClass, "value", "D");
    env->SetDoubleField(tdobj, jValueID, 2.3);
    return 0;
}

我想设置 class 'TestDouble' 的 'value' 的值， 'value' 的类型是 "Double" （不是双精度）。

E/dalvikvm: VM aborting
Fatal signal 6
Send stop signal to pid:5830 in void debuggerd_signal_handler(int, siginfo_t*, void*)

我只是把关键的错字贴在这里，很明显错误发生在：

env->SetDoubleField(tdobj, jValueID, 2.3);

那我该怎么做才能解决这个问题呢？非常感谢！

Answer 1

您的 C 代码假定 TestDouble.value 是原语 double 而不是对象 Double，因此在运行时失败。

您可以将 class 定义更改为

public class TestDouble
{
    public double value;
    public long num;
    public TestDouble(long num, double value)
    {
        this.num = num;
        this.value = value;
    }
}

Answer 2

我得到了正确答案：



JNIEXPORT jint JNICALL test_jni_Native_testSet(JNIEnv *env, jclass type, jobject tdobj)
{
    //Create Integer class, get constructor and create Integer object
    jclass intClass = env->FindClass(env, "java/lang/Integer");
   jmethodID initInt = env->GetMethodID(env, intClass, "<init>", "(I)V");
    if (NULL == initInt) return -1;
    jobject newIntObj = env->NewObject(env, intClass, initInt, 123);

//Now set your integer into value atttribute. For this, I would
//recommend you to have a java setter and call it in the same way 
//as shown above

//clean reference
env->DeleteLocalRef(env, newIntObj); 
return 0;
}

Integer/Double 和其他包装器类型可以用同样的方式处理..

Answer 3

如果您的 Double 对象已在 JAVA 代码中创建，则无需在 C++（抱歉，不是 C）部分创建新的 Double 对象。您可以执行以下操作：

void setDoubleObjectField(JNIEnv* env, jobject PropertyDoubleObj, char* FieldName, double dValue)
{
    jclass PropertyClass = (env)->GetObjectClass(PropertyDoubleObj);

    //printf("%s\n",getClassName( env, PropertyDoubleObj));

    jfieldID fidNumber = (env)->GetFieldID(PropertyClass, FieldName, "Ljava/lang/Double;");
    if (fidNumber == 0)
    {

        CreateException(env, std::string("Failed to query field from PropertyDouble class: ") + FieldName);
        return;
    }

    jobject DoubleObject = env->GetObjectField(PropertyDoubleObj, fidNumber);

    if (DoubleObject == 0) // That means, the object was simply not created. Just create one.
    {
        jclass DoubleClass = env->FindClass("java/lang/Double");
        jmethodID initDouble = env->GetMethodID( DoubleClass, "<init>", "(D)V");
        if (NULL == initDouble)
        {
            CreateException(env, "Failed to create Double object.");
            return;

        }
        DoubleObject = env->NewObject( DoubleClass, initDouble, dValue);
        env->SetObjectField(PropertyDoubleObj, fidNumber, DoubleObject);
        env->DeleteLocalRef(DoubleObject);
    }
    else
    {
        jclass DoubleCls = (env)->GetObjectClass(DoubleObject);
        jfieldID fidNumber_double = (env)->GetFieldID(DoubleCls, "value", "D");
        if (fidNumber_double == 0)
        {
            CreateException(env, "Failed to query value field from Double class.");
            return;
        }

        env->SetDoubleField(DoubleObject, fidNumber_double, dValue);
    }

}

如何通过 JNI 设置 Class 类型变量的 "Double" 值？

How can I set the value of "Double" type variable of my Class by JNI?

c++

java

java-native-interface

android

native