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Java Forum / General / November 2006What does FindClass() return?
Hallo, what is the return value of 'findClass' exactly? Is it a pointer to the memory which contains executable code of the class? Can I feed this pointer myself with a byte stream that contains a class file (*.class)? My problem is this, that I don't have any JAR file to load my classes from it, I get the executable class code in a byte stream and want call some methods of it... Hava anbody any idea? Thanks in advance, Anahita > what is the return value of 'findClass' exactly? Is > it a pointer to the memory which contains executable [quoted text clipped - 3 lines] > my classes from it, I get the executable class code > in a byte stream and want call some methods of it... As the signature says then it returns a Class object. The defineClass method can produce a class from a byte[]. Arne Hello, I tried with the function 'DefineClass(...)' but I get always a NULL pointer with the following exception: Exception in thread "main" java.lang.ClassFormatError: Unknown constant tag 0 in class file testClass 'testClass' is a very simple class without any methods, etc. Is my byte[] not correct? What is 'tag 0'?! Thanks in advance, Anahita > I tried with the function 'DefineClass(...)' but I get always a > NULL pointer with the following exception: [quoted text clipped - 3 lines] > 'testClass' is a very simple class without any methods, etc. > Is my byte[] not correct? What is 'tag 0'?! It looks like you are passing the source instead of the compiled class file. PS: btw, one cannot get both a NULL pointer and an exception. If an exception is thrown, then nothing is "return"ed. Hello, I copy the contents of my 'testClass.class' into a variable of type byte[]. I have created this class file with java compiler version '1.5.0'. About the exception: It is really so, that a get a NULL pointer. But I ask then JVM for occured Exceptions with the method 'ExceptionOccurred()' and if it retruns true (it is the case) I call 'ExceptionDescribe()' to get the message. Any idea?! Anahita > I copy the contents of my 'testClass.class' into a variable of type > byte[]. I have created this class file with java compiler version [quoted text clipped - 4 lines] > 'ExceptionOccurred()' and if it retruns true (it is the case) I call > 'ExceptionDescribe()' to get the message. For future reference, it would have been a good idea to say that you are using JNI, not Java. It wasn't until this post that it became clear to me which you were referring to, since java.lang.ClassLoader has methods whose names are similar or identical to the JNI functions you seem to be using. The replies you've gotten so far seem to assume you're referring to the ClassLoader methods. Instead of simply describing your problems, why don't you post the code that isn't working. For example, we cannot possibly guess if your byte[] or the call to DefineClass are correct, based only on your vague descriptions. /gordon  Signature[ don't email me support questions or followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e Hello, thank you for your advices! Here is my code and as you have seen I use JNI in C++ to execute Java code. ... // here I build my byte array, I read the contents // of a class file and add them to byte array ... typedef std::vector<char> BYTE_ARRAY ... BYTE_ARRAY byteField; FILE* f = fopen("D:\\Temp\\testClass.class", "r"); if (NULL != f) { char line[100]; while (NULL != fgets(line, 100, f)) { for (int i = 0; i < 100; ++i) byteField.push_back(line[i]); } } ... // try to define the class ... BYTE_ARRAY::size_type len = byteField.size(); jbyteArray jByteField = mEnv->NewByteArray(static_cast<jsize>(len)); jbyte* jBytes = mEnv->GetByteArrayElements(jByteField, 0); for (int i = 0; i < static_cast<int>(len); ++i) jBytes[i] = static_cast<jbyte>(byteField[i]); ... jclass jClass = mEnv->DefineClass("testClass", NULL, jBytes, len); if (mEnv->ExceptionOccurred()) mEnv->ExceptionDescribe(); ... jClass contains here NULL and the following exception has occured: Exception in thread "main" java.lang.ClassFormatError: Unknown constant tag 0 in class file testClass I hope that you have now sufficient information to see what is wrong. Thank you in advance, Anahita > thank you for your advices! Here is my code and as you have > seen I use JNI in C++ to execute Java code. > > FILE* f = fopen("D:\\Temp\\testClass.class", "r"); If you're on windoze platform, you need "rb", rather than "r" > char line[100]; > while (NULL != fgets(line, 100, f)) You'd better use fread (rather than fgets), since class files are not line-oriented, but binary. Unlike fgets, fread will also tell you the number of bytes actually read. (this is important at end of file, so you don't generate trailing garbage) > I hope that you have now sufficient information to > see what is wrong. yes, much better, now :-) > char line[100]; > while (NULL != fgets(line, 100, f)) > { > for (int i = 0; i < 100; ++i) > byteField.push_back(line[i]); > } Use fread(), not fgets(), to read your binary data. Also, your code assumes (probably incorrectly) that exactly 100 characters was read each time. Probably you have corrupted the class contents here. I'd suggest that you try writing the resulting array to a file and comparing it with the original before you attempt DefineClass(). Make this part work first! Note too that DefineClass() wants a jbyte* (i.e. native array of jbyte), not a "java byte array" as you're using in the subsequent code. /gordon Signature[ don't email me support questions or followups ] g o r d o n + n e w s @ b a l d e r 1 3 . s e Hello, thank you for usful hints! I could solve my problem, the reason was actually 'fgets(..)' and the corrupt byte array... Anahita Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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