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char *cFileName = JNU_GetStringNativeChars(env, fileName); if (cFileName) { /* call the real Win32 function */ result = (jint)CreateFile(cFileName, desiredAccess, shareMode, (SECURITY_ATTRIBUTES *)cSecAttrs, creationDistribution, flagsAndAttributes, (HANDLE)templateFile); free(cFileName); } /* else fall through, out of memory exception thrown */ if (secAttrs) { env->ReleaseIntArrayElements(secAttrs, cSecAttrs, 0); } return result; }
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First, we convert the security attributes stored in the int array into a jint array If the secAttrs argument is a NULL reference, we pass NULL as the security attribute to the Win32 CreateFile function Next, we call the utility function JNU_GetStringNativeChars ( 822) to obtain the le name represented as a locale-speci c C string Once we have converted the security attributes and le name, we pass the results of the conversions and the remaining arguments to the Win32 CreateFile function We take care to check for exceptions and release virtual machine resources (such as cSecAttrs) The Catol and Win32CreateFile examples demonstrate a common approach to writing wrapper classes and native methods Each native function (for example, CreateFile) maps to a single native stub function (for example, Java_Win32_CreateFile), which in turn maps to a single native method de nition (for example, Win32CreateFile) In one-to-one mapping, the stub function serves two purposes: 1 The stub adapts the native function s argument passing convention to what is expected by the Java virtual machine The virtual machine expects the native method implementation to follow a given naming convention and to accept two additional arguments (the JNIEnv pointer and the this pointer) 2 The stub converts between Java programming language types and native types For example, the Java_Win32_CreateFile function translates the jstring le name to a locale-speci c C string
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The one-to-one mapping approach requires you to write one stub function for each native function you want to wrap This becomes tedious when you are faced with the task of writing wrapper classes for a large number of native functions In this section we introduce the concept of shared stubs and demonstrate how shared stubs may be used to simplify the task of writing wrapper classes A shared stub is a native method that dispatches to other native functions The shared stub is responsible for converting the argument types from what is provided by the caller to what is accepted by the native functions We will soon introduce a shared stub class CFunction, but rst let us show how it can simplify the implementation of the Catol method:
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public class C { private static CFunction c_atol = new CFunction("msvcrtdll", // native library name "atol", // C function name "C"); // calling convention public static int atol(String str) { return c_atolcallInt(new Object[] {str}); } } Catol is no longer a native method (and thus no longer needs a stub function) Instead, Catol is de ned using the CFunction class The CFunction class internally implements a shared stub The static variable Cc_atol stores a CFunction object that corresponds to the C function atol in the msvcrtdll library (the multithreaded C library on Win32) The CFunction constructor call also speci es that atol follows the C calling convention ( 114) Once the c_atol eld is initialized, calls to the Catol method need only to redispatch through c_atolcallInt, the shared stub The CFunction class belongs to a class hierarchy that we will build up and use shortly: javalangObject CPointer
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Instances of the CFunction class denote a pointer to a C function CFunction is a subclass of CPointer, which denotes arbitrary C pointers:
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public class CFunction extends CPointer { public CFunction(String lib, // native library name String fname, // C function name String conv) { // calling convention } public native int callInt(Object[] args); }
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The callInt method takes as its argument an array of javalangObject It inspects the types of the elements in the array, converts them (from jstring to char *, for example), and passes them as arguments to the underlying C function The callInt method then returns the result of the underlying C function as an int The CFunction class can de ne methods such as callFloat or callDouble to handle C functions with other return types The CPointer class is de ned as follows:
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public abstract class CPointer { public native void copyIn( int bOff, // offset int[] buf, // source int off, // offset int len); // number public native void copyOut(); }
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