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Shutdown The virtual machine exits when one of the two criteria is met: All non-daemon threads terminate One of the executing threads invokes the exit() method of the Runtime or System classes. In this case, the security manager must grant exit privileges. As part of the virtual machine shutdown process, finalization may be run on any objects that have not already run their finalizers. Finalization is the process that allows objects to clean up any resources they may be using just prior to being garbage collected. By default, finalization does not run when the JVM is terminating; however, this can be changed by the executing application by invoking the System.runFinalizersOnExit(boolean) method and passing the value true. The ability for an application to use this method depends on the permissions granted by the security manager.
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Finalization is unreliable in that it is tied to the garbage collection process. There are no guarantees as to when the garbage collector will run or if a particular object will be collected. Even when garbage collection is explicitly requested using System.gc(), the garbage collector does not run immediately. The call to System.gc() simply requests garbage collection as soon as possible. This may never occur if other threads take priority. As a result, a resource such as a database connection or an I/O stream, will be tied up as the object that used the resource awaits finalization.
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Java Virtual Machine responsibilities In addition to managing classes and other lifecycle-related tasks, the JVM performs activities such as creating objects, garbage collection, security management and multithreading. Creating objects Objects are instances of a class that exist only at runtime. Classes are sometimes referred to as a template for creating objects. This processes is illustrated by figure 14.3.
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The class is loaded Byte code for the first class is located Is the class loaded Yes No
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Figure 14.3 The process used by the virtual machine to create objects at runtime. For each object, the class must first be loaded into the virtual machine and memory for the instance must be allocated. Once memory is allocated the object is initialized and the constructor is invoked.
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Objects are typically created using the new keyword or the static newInstance() method on the Class class. When an object is created, the JVM allocates enough memory for all the instance variables as well as all of the instance variables in all of the superclasses from which the object inherits. Once memory is allocated, the instance variables are initialized to their default values. If there is not enough memory available the object creation process aborts with an OutOfMemoryError. The next step that the JVM performs on a newly created object is to run the constructor specified by the creation statement. Regardless of the constructor that is invoked, a constructor for each superclass in the hierarchy must run. This can happen explicitly by referencing another constructor from within a constructor using the keywords this or super. However, if this or super is not specified, the default constructor is invoked implicitly by the JVM, thus guaranteeing that at least one constructor will execute within each superclass. Methods can be invoked during object initialization, both during the initialization of instance variables themselves and during the constructor execution. The JVM specifies no rules or limitations as to when a method can be invoked. Garbage collection and memory management One of the key features of the Java programming language is that the runtime environment manages memory allocation and deallocation automatically. This greatly eases the burden on developers from having to account for and track memory allocations in order to free memory once an object is no longer needed by the application. In some languages, such as C, programmers are required to allocate the proper amount of memory for an object and keep track of where this memory is so the memory block can be deallocated when the object is no longer needed. Failure to clean up unused memory results in what are known as memory leaks. If an application has a number of memory leaks, the application will slowly use up memory in the system. In cases where an application runs for long periods of time, and memory leaks are present, the system could begin to experience out-of-memory errors since there is no way to reclaim this memory. The only way to correct this problem is to terminate the application or, in some cases, reboot the system. In Java, the JVM performs the memory accounting activities in a manner that is invisible to developers. When an application requests that a Java object be created (e.g., new MyObject()), the virtual machine allocates the proper amount of memory required for the object as part of the construction process. Once the object is no longer used by the application it becomes eligible for garbage collection. The JVM detects classes that are no longer in use by periodically scanning object references in the heap. A Java object becomes eligible for garbage collection when all other objects in the system no longer hold a reference to it. The garbage collector itself runs on a low priority background thread and becomes active periodically throughout the lifecycle of the virtual machine. Garbage collection can be explicitly requested by an application with a call to Runtime.gc() or System.gc(). 412
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