ADDITIONAL JNI FEATURES in Java

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ADDITIONAL JNI FEATURES
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Re ection Support
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you should avoid complex synchronization and locking operations in
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JNI_OnUnload The JNI_OnUnload handler typically carries out simple tasks such
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as releasing the resources allocated by the native library The JNI_OnUnload handler runs when the class loader that loaded the library and all classes de ned by that class loader are no longer alive The JNI_OnUnload handler must not use these classes in any way In the above JNI_OnUnload de nition, you must not perform any operations that assume Class_C still refers to a valid class The DeleteWeakGlobalRef call in the example frees the memory for the weak global reference itself, but does not manipulate the referred class C in any way In summary, you should be careful when writing JNI_OnUnload handlers Avoid complex locking operations that may introduce deadlocks Keep in mind that classes have been unloaded when the JNI_OnUnload handler is invoked
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85 Re ection Support
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Re ection generally refers to manipulating language-level constructs at runtime For example, re ection allows you to discover at run time the name of arbitrary class objects and the set of elds and methods de ned in the class Re ection support is provided at the Java programming language level through the javalangreflect package as well as some methods in the javalangObject and javalangClass classes Although you can always call the corresponding Java API to carry out re ective operations, the JNI provides the following functions to make the frequent re ective operations from native code more ef cient and convenient: GetSuperclass returns the superclass of a given class reference IsAssignableFrom checks whether instances of one class can be used when instances of another class are expected GetObjectClass returns the class of a given jobject reference IsInstanceOf checks whether a jobject reference is an instance of a given class FromReflectedField and ToReflectedField allow the native code to convert between eld IDs and javalangreflectField objects They are new additions in Java 2 SDK release 12 FromReflectedMethod and ToReflectedMethod allow the native code to convert between method IDs, javalangreflectMethod objects and javalangreflectConstructor objects They are new additions in Java 2 SDK release 12 105
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JNI Programming in C++
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ADDITIONAL JNI FEATURES
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86 JNI Programming in C++
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The JNI presents a slightly simpler interface for C++ programmers The jnih le contains a set of de nitions so that C++ programmers can write, for example:
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jclass cls = env->FindClass("java/lang/String");
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instead of in C:
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jclass cls = (*env)->FindClass(env, "java/lang/String");
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The extra level of indirection on env and the env argument to FindClass are hidden from the programmer The C++ compiler inlines the C++ member function calls to their equivalent C counterparts; the resulting code is exactly the same There is no inherent performance difference between using the JNI in C or C++ In addition, the jnih le also de nes a set of dummy C++ classes to enforce the subtyping relationships among different jobject subtypes:
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// JNI reference types defined in C++ class _jobject {}; class _jclass : public _jobject {}; class _jstring : public _jobject {}; typedef _jobject* jobject; typedef _jclass* jclass; typedef _jstring* jstring;
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The C++ compiler is able to detect at compile time if you pass in, for example, a jobject to GetMethodID:
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// ERROR: pass jobject as a jclass: jobject obj = env->NewObject(); jmethodID mid = env->GetMethodID(obj, "foo", "()V");
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Because GetMethodID expects a jclass reference, the C++ compiler will give an error message In the C type de nitions for JNI, jclass is the same as jobject:
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typedef jobject jclass;
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Therefore, a C compiler is not able to detect that you have mistakenly passed a jobject instead of jclass
ADDITIONAL JNI FEATURES
JNI Programming in C++
The added type hierarchy in C++ sometimes necessitates additional casting In C, you can fetch a string from an array of strings and assign the result to a jstring:
jstring jstr = (*env)->GetObjectArrayElement(env, arr, i);
In C++, however, you need to insert an explicit conversion:
jstring jstr = (jstring)env->GetObjectArrayElement(arr, i);