![\"lib-dll-java\"](\"http:\/\/www.viaboxx.de\/wp-content\/uploads\/2014\/06\/lib-dll-java.jpg\")
<\/a><\/figure><\/div>\n\n\n\nThis blog post is about the interoperation of Java with a native DLL. It could have been created with Microsoft Visual Studio C++<\/a>. The DLL links the static link library and is a \u201cMulti-threaded DLL\u201d with \u201cNo Common Language Runtime support\u201d.<\/p>\n\n\n\n From the Java side, you have the choice between Java Native Interface (JNI)<\/a> and Java Native Access (JNA)<\/a>.<\/p>\n\n\n\n The usual low-level way is to use JNI to access the DLL, but this is much more error-prone and less productive than using JNA.<\/p>\n\n\n\n The JNA framework consists of a single jna.jar and can be downloaded here: Java Native Access (JNA) \u2013 downloads and documentation<\/a>. If you use maven to build the project, just add the dependency:<\/p>\n\n\n\n You basically create a Java interface to wrap the DLL functions to be invoked, but\u2026<\/p>\n\n\n\n Here are some technical details of interesting integration problems, depending on the signature of the DLL functions, that need to be invoked:<\/p>\n\n\n\n Consider the calling convention<\/strong><\/p>\n\n\n\n Depending on the calling convention<\/a> that the DLL expects, the wrapper interface needs to inherit from \u201ccom.sun.jna.Library\u201d or \u201ccom.sun.jna.win32.StdCallLibrary\u201d.<\/p>\n\n\n\n The Microsoft-specific directive __declspec(dllexport) exports the functions for the DLL so that a client can invoke them. Usually, you create a macro definition: #define EXAMPLE_DLL<\/strong> to make the code more readable.<\/p>\n\n\n\n Data type mapping between C and Java<\/strong><\/p>\n\n\n\n The function \u201creturnDLLVersion\u201d returns a char pointer (C-String). The JNA-framework converts this to a Java String (Unicode). It offers automatic conversions for most C-data types to\/from Java. Detailed documentation is here: \u201cMapping between Java and Native<\/a>\u201c.<\/p>\n\n\n\n Writing output into shared memory<\/strong><\/p>\n\n\n\n The C-function \u201creadBufferContent()\u201d writes an output string into memory, that Java needs to provide and read. If the maximum output length is known before, you can use this procedure:<\/p>\n\n\n\n An output parameter of type \u201cint\u201d: call-by-reference<\/strong><\/p>\n\n\n\n The consolidate()-function demonstrates multiple problems:<\/p>\n\n\n\n Dynamic memory reserved in the DLL: return an array of struct of dynamic length<\/strong><\/p>\n\n\n\n The array elements are C-structs of type \u201cResultStruct\u201d. Unfortunately, JNA cannot automatically convert an array of structs. It can convert a single struct into a Java object and arrays of primitives, so here the conversion must be implemented ourselves.<\/p>\n\n\n\n There are two questions:<\/p>\n\n\n\n The length and thus the amount of memory is unknown before the function call, so we decide not to use a fixed-length Memory object of JNA. Instead, the DLL dynamically reserves memory, but we must pay attention to the scope so that the memory is still allocated when the function-call returns. The C++ program can use \u201cmalloc()\u201d or \u201cnew\u201d to allocate heap memory. To free it correctly, the DLL provides a separate freeMemory() function:<\/p>\n\n\n\n \n net.java.dev.jna\n jna\n 3.2.7\n compile\n<\/pre>\n\n\n\n
C++:\n#define EXAMPLE_DLL extern \"C\" __declspec(dllexport)\nEXAMPLE_DLL char* returnDLLVersion();<\/pre>\n\n\n\n
Java:\npublic interface ExampleDLL extends Library {\n ExampleDLL INSTANCE = (ExampleDLL) Native.loadLibrary(\"example.dll\", ExampleDLL.class);\n String returnDLLVersion();\n}<\/pre>\n\n\n\nC++:\nEXAMPLE_DLL bool readBufferContent(unsigned char* outbuf);\n\nJava:\nboolean readBufferContent(Memory memory);<\/pre>\n\n\n\n
Memory memory = new Memory(100);<\/li>
ExampleDLL.INSTANCE.readBufferContent(memory);<\/li>
String bufferContent = memory.getString(0);<\/li>C++:\nEXAMPLE_DLL ResultStruct* consolidate(const short *cardTypes, int *numberOfResults);\n\nJava:\nPointer consolidate(int[] cardTypes, IntByReference numberOfResults);<\/pre>\n\n\n\n
Java can get the int-value after the call with:
int value = numberResults.getValue();<\/li>C++:\nvoid freeMemory(ResultStruct *arr) { if(arr) delete[] arr; }\n\nJava:\nvoid freeMemory(Pointer p);<\/pre>\n\n\n\n