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function, it passes the underlying string object to the caller of Cf and then destroys the local reference that was originally created by the JNI function NewObject Local references are also only valid in the thread that creates them A local reference that is created in one thread cannot be used in another thread It is a programming error for a native method to store a local reference in a global variable and expect another thread to use the local reference 512 Global References You can use a global reference across multiple invocations of a native method A global reference can be used across multiple threads and remains valid until it is freed by the programmer Like a local reference, a global reference ensures that the referenced object will not be garbage collected Unlike local references, which are created by most JNI functions, global references are created by just one JNI function, NewGlobalRef The following version of MyNewString illustrates how to use a global reference We highlight the differences between the code below and the code that mistakenly cached a local reference in the last section:
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/* This code is OK */ jstring MyNewString(JNIEnv *env, jchar *chars, jint len) { static jclass stringClass = NULL; if (stringClass == NULL) { jclass localRefCls = (*env)->FindClass(env, "java/lang/String"); if (localRefCls == NULL) { return NULL; /* exception thrown */ } /* Create a global reference */ stringClass = (*env)->NewGlobalRef(env, localRefCls); /* The local reference is no longer useful */ (*env)->DeleteLocalRef(env, localRefCls); /* Is the global reference created successfully */ if (stringClass == NULL) { return NULL; /* out of memory exception thrown */ } } }
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Weak Global References
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The modi ed version passes the local reference returned from FindClass to which creates a global reference to the javalangString class object We check whether the NewGlobalRef has successfully created stringClass after deleting localRefCls because the local reference localRefCls needs to be deleted in either case
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NewGlobalRef,
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513 Weak Global References Weak global references are new in Java 2 SDK release 12 They are created using NewGlobalWeakRef and freed using DeleteGlobalWeakRef Like global references, weak global references remain valid across native method calls and across different threads Unlike global references, weak global references do not keep the underlying object from being garbage collected The MyNewString example has shown how to cache a global reference to the javalangString class The MyNewString example could alternatively use a weak global reference to store the cached javalangString class It does not matter whether we use a global reference or a weak global reference because javalangString is a system class and will never be garbage collected Weak global references become more useful when a reference cached by the native code must not keep the underlying object from being garbage collected Suppose, for example, a native method mypkgMyClsf needs to cache a reference to the class mypkgMyCls2 Caching the class in a weak global reference allows the mypkgMyCls2 class to still be unloaded:
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JNIEXPORT void JNICALL Java_mypkg_MyCls_f(JNIEnv *env, jobject self) { static jclass myCls2 = NULL; if (myCls2 == NULL) { jclass myCls2Local = (*env)->FindClass(env, "mypkg/MyCls2"); if (myCls2Local == NULL) { return; /* can t find class */ } myCls2 = NewWeakGlobalRef(env, myCls2Local); if (myCls2 == NULL) { return; /* out of memory */ } } /* use myCls2 */ }
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We assume that MyCls and MyCls2 have the same lifetime (For example, they may be loaded by the same class loader) Thus we do not consider the case when 65
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Comparing References
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MyCls2 is unloaded and later reloaded while MyCls and its native method implementation Java_mypkg_MyCls remain to be in use If that could happen, we would have to check whether the cached weak reference still points to a live class object or points to a class object that has already been garbage collected The next section will explain how to perform such checks on weak global references
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514 Comparing References Given two local, global, or weak global references, you can check whether they refer to the same object using the IsSameObject function For example:
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(*env)->IsSameObject(env, obj1, obj2)
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returns JNI_TRUE (or 1) if obj1 and obj2 refer to the same object, and returns JNI_FALSE (or 0) otherwise A NULL reference in JNI refers to the null object in the Java virtual machine If obj is a local or a global reference, you may use either
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