Confusing IDs with References in Java

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106 Confusing IDs with References
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The JNI exposes objects as references Classes, strings, and arrays are special types of references The JNI exposes methods and elds as IDs An ID is not a reference Do not call a class reference a class ID or a method ID a method reference
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References are virtual machine resources that can be managed explicitly by native code The JNI function DeleteLocalRef, for example, allows native code to delete a local reference In contrast, eld and method IDs are managed by the virtual machine and remain valid until their de ning class is unloaded Native code cannot explicitly delete a eld or method ID before the the virtual machine unloads the de ning class Native code may create multiple references that refer to the same object A global and a local reference, for example, may refer to the same object In contrast, a unique eld or method ID is derived for the same de nition of a eld or a method If class A de nes method f and class B inherits f from A, the two GetMethodID calls in the following code always return the same result:
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jmethodID MID_A_f = (*env)->GetMethodID(env, A, "f", "()V"); jmethodID MID_B_f = (*env)->GetMethodID(env, B, "f", "()V");
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107 Caching Field and Method IDs
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Native code obtains eld or method IDs from the virtual machine by specifying the name and type descriptor of the eld or method as strings ( 41, 42) Field and method lookups using name and type strings are slow It often pays off to cache the IDs Failure to cache eld and method IDs is a common performance problem in native code In some cases caching IDs is more than a performance gain A cached ID may be necessary to ensure that the correct eld or method is accessed by native code The following example illustrates how the failure to cache a eld ID can lead to a subtle bug:
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class C { private int i; native void f(); }
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Suppose that the native method f needs to obtain the value of the eld i in an instance of C A straightforward implementation that does not cache an ID accomplishes this in three steps: 1) get the class of the object; 2) look up the eld ID for i from the class reference; and 3) access the eld value based on the object reference and eld ID:
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// No field IDs cached JNIEXPORT void JNICALL Java_C_f(JNIEnv *env, jobject this) { jclass cls = (*env)->GetObjectClass(env, this); /* error checking */ jfieldID fid = (*env)->GetFieldID(env, cls, "i", "I"); /* error checking */ ival = (*env)->GetIntField(env, this, fid); /* ival now has the value of thisi */ }
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The code works ne until we de ne another class D as a subclass of C, and declare a private eld also named i in D:
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// Trouble in the absence of ID caching class D extends C { private int i; D() { f(); // inherited from C } }
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When D s constructor calls Cf, the native method receives an instance of D as the this argument, cls refers to the D class, and fid represents Di At the end of the native method, ival contains the value of Di, instead of Ci This might not be what you expected when implementing native method Cf The solution is to compute and cache the eld ID at the time when you are certain that you have a class reference to C, not D Subsequent accesses from this cached ID will always refer to the right eld Ci Here is the corrected version:
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// Version that caches IDs in static initializers class C { private int i; native void f(); private static native void initIDs(); static { initIDs(); // Call an initializing native method } }
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The modi ed native code is:
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