Intercept native methods

hello,

I want to change the data type returns, Is it possible ?
Can I do this by intercepting a native method ?? if yes please shed some light.

Thanks

What do you mean by 'change the data type returns'?
You could wrap the existing native calls and do whatever you want.

Quote:

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Originally Posted by Tolls

What do you mean by 'change the data type returns'?
You could wrap the existing native calls and do whatever you want.

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one of the native functions returns a string for a stream of bytes. If a zero exists the returned string will be incomplete. So i am thinking to make return type char[] or byte[].
So how can i get around this issue ?
How do i do the wrapping ?
Thanks

Why is your native function returning a String if it should be a byte[]?
Has the Java side been mis-translated?

Quote:

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Originally Posted by Tolls

Why is your native function returning a String if it should be a byte[]?
Has the Java side been mis-translated?

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i dont know..it is a 3rd party package i am dealing with.
Any ideas how to get round it. ?
Thanks

So all you can do is wrap the call in something that'll translate it.
If the 3rd party stuff returns a String and you want a byte[] then:

Code:

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public byte[] wrappedMethod() {
  String result = nativeMethodCall();
  return result.getBytes(<whatever character set the String is in>);
}

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Quote:

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Originally Posted by Tolls

Code:

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  String result = nativeMethodCall();

---

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Thanks for the reply.
The nativeMethodCall() above in first place returns a String which means the byte stream will be incomplete if a 0x00 is met. is that right ? this is the problem i am trying to solve. hints?
I do have access to the binary file and i can tell the bytes returned are incomplete.

Thanks

If the native method returns a String wirth 0x00 in it then that is what you will receive.
Just look at the bytes in Java, as above.

Quote:

---

Originally Posted by Tolls

If the native method returns a String wirth 0x00 in it then that is what you will receive.
Just look at the bytes in Java, as above.

---

Hello

I tried the byte[] but no joy.
My initial thinking was that: Since the String in Java wrapper classes correspond to a char* pointer in native functions then I believe there should be a way to get the full string pointed to.
What do you think ?
Thanks

Which is what is contained in the String.
It wouldn't work otherwise.
What is the value you are getting back in the String, when printing out the bytes?

What is happening when you try and get the byte[]?
You haven't said.

Quote:

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Originally Posted by Tolls

Which is what is contained in the String.
It wouldn't work otherwise.
What is the value you are getting back in the String, when printing out the bytes?

What is happening when you try and get the byte[]?
You haven't said.

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Hello,

I did manage to do the bytes by changing the return value of the native method from string to byte[];
But the value is the again the incomplete string e.g. 11 22 33 00 where it should be 11 22 33 00 AA FF.
This means that the native function is returning a string (zero terminated).
Hints?
Thanks

So, complain to whoever wrote that code.
You are calling a native method which is returning the wrong thing, at least according to you.
The only solution we can give to that is to get someone to rewrite that method.

Will this library be beneficial in my case ?
Thanks for your effort

"Pure Java agent to intercept native method calls to handle their implementation in Java code rather than native code."
That says it is for redirecting calls to a native method so that they go to a Java method instead.
So unless you want to rewrite what this 3rd party package does then this is not what you want.

Honestly, you need to talk to whoever wrote this, or see what the documentation says about its interaction with Java.

Hello,

I want to double check what i am doing is correct.
If in a Java package there is a TestImp.java and has the method:

Code:

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public native String getTestValue()
 throws Exception;

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and in the dll files there is an exported function called package_imp_getTestValue.
So how the java method is mapped to the dll exported function ? is there a wrapper in between ?


Thanks

You can jump around the original native method all you want, but it returns whatever it returns and if it's a zero terminated String then that's what it returns, no matter how you call the type of the returned value. If you want zeros in your String (or char array or whatever) you have to change the original native method. There's no magic involved.

kind regards,

Jos

Ok thanks ;)
could you please answer me this

Quote:

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So how the java method is mapped to the dll exported function ? is there a wrapper in between ?

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Thanks

Quote:

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Originally Posted by raedbenz

Ok thanks ;)
could you please answer me this
Thanks

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If you're using JNI, most likely there's a small wrapper involve that trampolines to the final native code method.

kind regards,

Jos

You've definitely got a jumping metaphor going at the moment, Jos.

Quote:

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Originally Posted by Tolls

You've definitely got a jumping metaphor going at the moment, Jos.

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Don't jump to conclusions ...

kind regards,

Jos ;-)