Swig：将返回类型std :: string（二进制）转换为java byte [] [英] Swig: convert return type std::string(binary) to java byte[]

问题描述

我的情况是我有一个C ++类（MyClass），其方法具有以下签名：

My situation is that i have a C++ class (MyClass) with a method that has the following signature:

bool getSerialized(const stdString & name, std::string & serialized);

其中name是参数，序列化是out参数。

Where name is a in argument and serialized is an out argument.

我通过在'i'文件中进行％extend和％ignore声明来实现它，如下所示：

I got it working by making a %extend and %ignore declarations in the 'i' file as follows:

%extend MyClass{
    std::string getSerialized(const std::string & name){
        std::string res;
        $self->getSerialized(name, res);
        return res;
};
%rename("$ignore", fullname=1) "MyClass::getSerialized";

因此可以使用Java中的方法：

So the method con be used from Java like:

MyClass mc = new MyClass();
String res = mc.getSerialized("test");

但是现在我遇到了一个问题，序列化的std :: string包含二进制数据，包括' \ 0'字符女巫表示C字符串的结尾，实际上下面的代码显示了C ++中的问题：

But now i have encountered a problem, the serialized std::string contains binary data, including the '\0' character witch indicates the end of a C String, in fact the following code shows the problem in C++:

std::string s;
s.push_back('H');
s.push_back('o');
s.push_back(0);
s.push_back('l');
s.push_back('a');
std::cout << "Length of std::string " << s.size() << std::endl;
std::cout << "CString: '" << s.c_str() << "'" << std::endl;

上面的代码显示：

Length of std::string 5
CString: 'Ho'

正如我在SWIG生成的包装文件中看到的那样，wrap方法实际上调用c_str（），包装代码：

As i have seen in the wrap file generated by SWIG, the wrap method actually calls c_str(), code of wrap:

jstring jresult = 0 ;
std::string result;
result = (arg1)->getSerialized();
jresult = jenv->NewStringUTF((&result)->**c_str()**); 
return jresult;

正如预期的那样，Java中收到的字符串会被截断。那么我怎么能改变（大概）我的％扩展函数包装器，所以我可以将它作为字节数组（byte []）返回，而不需要事先知道数组的长度。如果可以在SWIG层中创建byteArray会很棒，所以我可以从Java调用方法，如：

So as expected the received String in Java gets truncated. So how can i change (presumably) my %extend function wrapper so i can return this as a byte array (byte[]), without previously knowing the length of the array. It would be great if the byteArray could be created in the SWIG layer, so i could invoke the method from Java like:

byte[] serialized = mc.getSerialized("test");

其他注意事项：
给出了使用std :: string存储二进制数据的方法，使用Google protobuf库的返回类型 C ++ protobuf usage

有一个非常类似的问题，包括tittle Swig：将返回类型std :: string转换为java byte [] ，但二进制数据没有任何情况，所以那里给出的解决方案没有在这里申请。

There is a very similar question, including the tittle Swig: convert return type std::string to java byte[] but there is no case for binary data, so the solution given there doesn't apply here.

使用SWIG 2.

推荐答案

你可以做您正在尝试使用一些类型映射和一些JNI。我把一个例子放在一起：

You can do what you're trying to do with a few typemaps and some JNI. I put together an example:

%module test

%include <std_string.i>

%typemap(jtype) bool foo "byte[]"
%typemap(jstype) bool foo "byte[]"
%typemap(jni) bool foo "jbyteArray"
%typemap(javaout) bool foo { return $jnicall; }
%typemap(in, numinputs=0) std::string& out (std::string temp) "$1=&temp;"
%typemap(argout) std::string& out {
  $result = JCALL1(NewByteArray, jenv, $1->size());
  JCALL4(SetByteArrayRegion, jenv, $result, 0, $1->size(), (const jbyte*)$1->c_str());
}
// Optional: return NULL if the function returned false
%typemap(out) bool foo {
  if (!$1) {
    return NULL;
  }
}

%inline %{
struct Bar {
  bool foo(std::string& out) {
    std::string s;
    s.push_back('H');
    s.push_back('o');
    s.push_back(0);
    s.push_back('l');
    s.push_back('a');
    out = s;
    return true;
  }
};
%}

它声明C ++包装器为函数返回一个Java字节数组匹配 bool foo 。它还设置一个临时的 std :: string 来提供隐藏输入参数的 foo 的实际实现Java接口本身。

It states that the C++ wrapper with return a Java byte array for the functions that match bool foo. It also sets up a temporary std::string to give to the real implementation of foo that hides the input parameter from the Java interface itself.

一旦调用完成，它就会创建并返回一个字节数组，前提是函数没有返回false。

Once the call has been made it creates and returns a byte array provided the function didn't return false.

我检查了它是否按预期工作：

I checked that it all worked as expected with:

public class run { 
  public static void main(String[] argv) {
    String s = "ho\0la";
    System.out.println(s.getBytes().length);

    System.loadLibrary("test");
    Bar b = new Bar();
    byte[] bytes = b.foo();
    s = new String(bytes);
    System.out.println(s + " - " + s.length());
    assert(s.charAt(2) == 0);
  }
}

你应该知道演员阵容的含义返回类型为 c_str（）的 const jbyte * - 它可能并不总是你想要的。

You should be aware of the implications of the cast to const jbyte* from the return type of c_str() - it may not always be what you wanted.

作为替代方案，如果输出字节数组的大小实际上是固定的或平凡可预测的，您可以将其作为输入开始预先分配。这可以工作，因为数组首先通过引用有效地传递给函数。

As an alternative if the size of the output byte array was actually fixed or trivially predictable you could pass that in pre-allocated as the input to begin with. This would work because arrays are effectively passed by reference into functions in the first place.

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