Load and Unload Handlers in Java

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Load and Unload Handlers
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from the library (thus there is no need to declare the function using JNIEXPORT) The native function g_impl must still, however, follow the JNICALL calling convention The RegisterNatives function is useful for a number of purposes: It is sometimes more convenient and more ef cient to register a large number of native method implementations eagerly, as opposed to letting the virtual machine link these entries lazily You may call RegisterNatives multiple times on a method, allowing the native method implementation to be updated at runtime RegisterNatives is particularly useful when a native application embeds a virtual machine implementation and needs to link with a native method implementation de ned in the native application The virtual machine would not be able to nd this native method implementation automatically because it only searches in native libraries, not the application itself
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84 Load and Unload Handlers
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Load and unload handlers allow the native library to export two functions: one to be called when SystemloadLibrary loads the native library, the other to be called when the virtual machine unloads the native library This feature was added in Java 2 SDK release 12 841 The JNI_OnLoad Handler When SystemloadLibrary loads a native library, the virtual machine searches for the following exported entry in the native library:
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JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *jvm, void *reserved);
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You can invoke any JNI functions in an implementation of JNI_Onload A typical use of the JNI_OnLoad handler is caching the JavaVM pointer, class references, or method and eld IDs, as shown in the following example:
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JavaVM *cached_jvm; jclass Class_C; jmethodID MID_C_g;
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The JNI_OnLoad Handler
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JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *jvm, void *reserved) { JNIEnv *env; jclass cls; cached_jvm = jvm; /* cache the JavaVM pointer */ if ((*jvm)->GetEnv(jvm, (void **)&env, JNI_VERSION_1_2)) { return JNI_ERR; /* JNI version not supported */ } cls = (*env)->FindClass(env, "C"); if (cls == NULL) { return JNI_ERR; } /* Use weak global ref to allow C class to be unloaded */ Class_C = (*env)->NewWeakGlobalRef(env, cls); if (Class_C == NULL) { return JNI_ERR; } /* Compute and cache the method ID */ MID_C_g = (*env)->GetMethodID(env, cls, "g", "()V"); if (MID_C_g == NULL) { return JNI_ERR; } return JNI_VERSION_1_2; }
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The JNI_OnLoad function rst caches the JavaVM pointer in the global variable cached_jvm It then obtains the JNIEnv pointer by calling GetEnv It nally loads the C class, caches the class reference, and computes the method ID for Cg The JNI_OnLoad function returns JNI_ERR ( 124) on error and otherwise returns the JNIEnv version JNI_VERSION_1_2 needed by the native library We will explain in the next section why we cache the C class in a weak global reference instead of a global reference Given a cached JavaVM interface pointer it is trivial to implement a utility function that allows the native code to obtain the JNIEnv interface pointer for the current thread ( 814) :
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JNIEnv *JNU_GetEnv() { JNIEnv *env; (*cached_jvm)->GetEnv(cached_jvm, (void **)&env, JNI_VERSION_1_2); return env; }
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The JNI_OnUnload Handler
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842 The JNI_OnUnload Handler Intuitively, the virtual machine calls the JNI_OnUnload handler when it unloads a JNI native library This is not precise enough, however When does the virtual machine determine that it can unload a native library Which thread runs the JNI_OnUnload handler The rules of unloading native libraries are as follows: The virtual machine associates each native library with the class loader L of the class C that issues the SystemloadLibrary call The virtual machine calls the JNI_OnUnload handler and unloads the native library after it determines that the class loader L is no longer a live object Because a class loader refers to all the classes it de nes, this implies that C can be unloaded as well The JNI_OnUnload handler runs in a nalizer, and is either invoked synchroniously by javalangSystemrunFinalization or invoked asynchronously by the virtual machine Here is the de nition of a JNI_OnUnload handler that cleans up the resources allocated by the JNI_OnLoad handler in the last section:
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JNIEXPORT void JNICALL JNI_OnUnload(JavaVM *jvm, void *reserved) { JNIEnv *env; if ((*jvm)->GetEnv(jvm, (void **)&env, JNI_VERSION_1_2)) { return; } (*env)->DeleteWeakGlobalRef(env, Class_C); return; }
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The JNI_OnUnload function deletes the weak global reference to the C class created in the JNI_OnLoad handler We need not delete the method ID MID_C_g because the virtual machine automatically reclaims the resources needed to represent C s method IDs when unloading its de ning class C We are now ready to explain why we cache the C class in a weak global reference instead of a global reference A global reference would keep C alive, which in turn would keep C s class loader alive Given that the native library is associated with C s class loader L, the native library would not be unloaded and JNI_OnUnload would not be called The JNI_OnUnload handler runs in a nalizer In contrast, the JNI_OnLoad handler runs in the thread that initiates the SystemloadLibrary call Because JNI_OnUnload runs in an unknown thread context, to avoid possible deadlocks, 104
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