[英]how to write and read from bytebuffer passing from java to jni
I need help with my android project. 我的android项目需要帮助。 I want to pass a buffer from java to jni, and my C++ code will populate the data.
我想将缓冲区从java传递给jni,我的C ++代码将填充数据。 Then java will display them on the screen.
然后java将在屏幕上显示它们。 I am not familiar much with C++ and have no idea how to write to write to the buffer.
我对C ++并不熟悉,也不知道如何写入写入缓冲区。
this is what i got. 这就是我得到的。 in java
在java中
ByteBuffer bb = ByteBuffer.allocateDirect(216);
IssmJni.processBuffer(bb);
native method 本地方法
public static native void processBuffer(ByteBuffer bb);
I don't use jni_onload, so no javah in C++ 我不使用jni_onload,所以在C ++中没有javah
static void fillBuffer(JNIEnv *env, jclass clazz, jobject buf)
{
double *dBuf = env->GetDirectBufferAddress(env, buf);
}
I am stuck here, can I do double dbuf? 我被困在这里 ,我可以做双dbuf吗? or is it has to be a char ?
或者它必须是一个炭 ?
Let said I want to write 1,2,3,4,5 to this dbuf, how do I do it? 我想说我想给这个dbuf写1,2,3,4,5,我该怎么做? i am thinking of dbuf.put(1);
我在想dbuf.put(1); ... dbuf.put(5) but it is not working.
... dbuf.put(5)但它不起作用。 and after populating it, do I just call bb.get(position) in java?
在填充之后,我只是在java中调用bb.get(position)吗?
Someone clarify this for me please, example would be appreciated Thank you 有人为我澄清这个,例如将不胜感激谢谢
this is my table of native methods 这是我的原生方法表
static JNINativeMethod method_table[] = { static JNINativeMethod method_table [] = {
{"fac" , "(J)J" , (void *) factorial},
{"getBuffer", "()[D" , (void *) getBufferNative},
//{"processBuffer", "(Ljava/nio/ByteBuffer)V", (void *) fillBuffer}};
The other two works fine except the last one. 除了最后一个之外,其他两个工作正常。
I don't use jni_onload, so no javah in C++
我不使用jni_onload,所以在C ++中没有javah
It may be irrelevant to your question, but you sort of have to do one or the other. 这可能与您的问题无关,但您必须做一个或另一个。 You either need to do your Java-to-Native method mapping in the onLoad function or use javah to generate the method signatures that JNI can pick up at runtime.
您需要在onLoad函数中执行Java-to-Native方法映射,或者使用javah生成JNI可以在运行时获取的方法签名。 Otherwise your native methods will never be invoked.
否则,永远不会调用您的本机方法。
If your native method is properly being called, however, whatever you are doing is probably fine. 但是,如果正确调用了本机方法,那么无论你做什么都可能没问题。
I am stuck here, can I do double dbuf?
我被困在这里,我可以做双dbuf吗? or is it has to be a char?
或者它必须是一个炭?
The return value of the function is actually a void*
, so you can cast it to whatever pointer type you want since it is just a memory address. 该函数的返回值实际上是一个
void*
,因此您可以将它转换为您想要的任何指针类型,因为它只是一个内存地址。 However, the actual data type of the direct buffer will be jbyte
, which is a typedef of signed char
, so the following is probably best: 但是,直接缓冲区的实际数据类型将是
jbyte
,它是signed char
的typedef,因此以下可能是最好的:
jbyte *dBuf = env->GetDirectBufferAddress(env, buf);
Let said I want to write 1,2,3,4,5 to this dbuf, how do I do it?
我想说我想给这个dbuf写1,2,3,4,5,我该怎么做?
The notation in C/C++ to access values in an array is with [] notations, ie: C / C ++中用于访问数组中值的符号是[]符号,即:
jbyte *dBuf = env->GetDirectBufferAddress(env, buf);
dBuf[0] = 63;
dBuf[1] = 127;
// ...and so on...
and after populating it, do I just call bb.get(position) in java?
在填充之后,我只是在java中调用bb.get(position)吗?
Yes, you can use the accessor methods on ByteBuffer
to access the data you have written from your native code. 是的,您可以使用
ByteBuffer
上的访问器方法来访问您从本机代码中编写的数据。
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