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static jfieldID FID_C_i; JNIEXPORT void JNICALL Java_C_initIDs(JNIEnv *env, jclass cls) { /* Get IDs to all fields/methods of C that native methods will need */ FID_C_i = (*env)->GetFieldID(env, cls, "i", "I"); } JNIEXPORT void JNICALL Java_C_f(JNIEnv *env, jobject this) { ival = (*env)->GetIntField(env, this, FID_C_i); /* ival is always Ci, not Di */ }
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The eld ID is computed and cached in C s static initializer This guarantees that the eld ID for Ci will be cached, and thus the native method implementation Java_C_f will read the value of Ci independent of the actual class of the this object Caching may be needed for some method calls as well If we change the above example slightly so that classes C and D each have their own de nition of a private method g, f needs to cache the method ID of Cg to avoid accidentally calling Dg Caching is not needed for making correct virtual method calls Virtual methods by de nition dynamically bind to the instance on which the method is invoked Thus you can safely use the JNU_CallMethodByName utility function ( 623) to call virtual methods The previous example tells us, however, why we do not de ne a similar JNU_GetFieldByName utility function
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Unicode strings obtained from GetStringChars or GetStringCritical are not NULL-terminated Call GetStringLength to nd out the number of 16-bit Unicode characters in a string Some operating systems, such as Windows NT, expect two trailing zero byte values to terminate Unicode strings You cannot pass the result of GetStringChars to Windows NT APIs that expect a Unicode string You must make another copy of the string and insert the two trailing zero byte values
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The JNI does not enforce class, eld, and method access control restrictions that can be expressed at the Java programming language level through the use of mod137
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i ers such as private and final It is possible to write native code to access or modify elds of an object even though doing so at the Java programming language level would lead to an IllegalAccessException JNI s permissiveness was a conscious design decision, given that native code can access and modify any memory location in the heap anyway Native code that bypasses source-language-level access checks may have undesirable effects on program execution For example, an inconsistency may be created if a native method modi es a final eld after a just-in-time (JIT) compiler has inlined accesses to the eld Similarly, native methods should not modify immutable objects such as elds in instances of javalangString or javalangInteger Doing so may lead to breakage of invariants in the Java platform implementation
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Strings in the Java virtual machine consist of Unicode characters, whereas native strings are typically in a locale-speci c encoding Use utility functions such as JNU_NewStringNative ( 821) and JNU_GetStringNativeChars ( 822) to translate between Unicode jstrings and locale-speci c native strings of the underlying host environment Pay special attention to message strings and le names, which typically are internationalized If a native method gets a le name as a jstring, the le name must be translated to a native string before being passed to a C library routine The following native method, MyFileopen, opens a le and returns the le descriptor as its result:
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JNIEXPORT jint JNICALL Java_MyFile_open(JNIEnv *env, jobject self, jstring name, jint mode) { jint result; char *cname = JNU_GetStringNativeChars(env, name); if (cname == NULL) { return 0; } result = open(cname, mode); free(cname); return result; }
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We translate the jstring argument using the JNU_GetStringNativeChars function because the open system call expects the le name to be in the localespeci c encoding
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