How to use garbage collector in Java

Java Garbage CollectorTips for usage. The Garbage Collector (GC) is an important part of modern software virtual machine technology. Its design has an important impact on the performance of applications running on virtual machines. The impact is huge. Both the Java Virtual Machine (JVM) and the .net framework provide this functionality. Below we briefly introduce the principles of the garbage collector in the Java virtual machine.

Java's memory management is actually the management of objects, including the allocation and release of objects.

For programmers, use the new keyword to allocate objects; when releasing an object, just assign all references to the object to null. For GC, when the programmer creates an object, the GC begins to monitor the address, size and usage of the object. Usually, GC uses a directed graph to record and manage all objects in the heap. In this way it is determined which objects are "reachable" and which objects are "unreachable". When the GC determines that some objects are "unreachable", the GC is responsible for reclaiming these memory spaces.

GC is usually implemented by one or a group of processes in the JVM. It also occupies the heap space like the user program and also occupies the CPU when running. While the GC process is running, the application stops running.

Therefore, when the GC running time is long, the user can feel the pause of the Java program. On the other hand, if the GC running time is too short, the object recovery rate may be too low, which means there are still many things that should be The recycled objects are not recycled and still occupy a lot of memory. Therefore, when designing GC, a trade-off must be made between pause time and recovery rate.

According to the working principle of GC, we can use some techniques and methods to make GC operation more efficient and more in line with the requirements of the application.

The following are some suggestions for programming.

1. The most basic suggestion is to release references to useless objects as soon as possible.

When most programmers use temporary variables, they let the reference variable be automatically set to null after exiting the active scope (scope).

2. Use the finalize function as little as possible. The finalize function provides Java programmers with an opportunity to release objects or resources. However, it will increase the workload of the GC, so use finalize as little as possible to recover resources.

3. If you need to use frequently used pictures, you can use the soft application type. It can save the picture in the memory as much as possible for the program to call without causing OutOfMemory.

4. Pay attention to collection data types, including arrays, trees, graphs, linked lists and other data structures. For such objects, GC recycling is generally less efficient. If your program allows it, set unused reference objects to null as early as possible. This can speed up the work of GC. Therefore, unless necessary, the data structures used in the program should be as simple as possible.

5. When the program has a certain waiting time, the programmer can manually execute System.gc() to notify the GC to run, but the Java language specification does not guarantee that the GC will be executed.

The execution of the garbage collector in a JAVA program is automatic and cannot be forced. Even if the programmer can clearly judge that a piece of memory is useless and should be recycled, the programmer cannot force it. The garbage collector reclaims the memory block.

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