Java Foundation-JVM Memory Recycling

Sun's jvmgenerationalcollecting (garbage collection) principle is that it divides objects into younger generations (young), older generations (tenured), and persistent generations (Perm), using different algorithms for objects of different lifecycles. (Based on the object life cycle analysis)

1.Young (Young Generation)

The young generation is divided into three districts. One Eden area, two survivor districts. Most objects are generated in the Eden area. When the Eden Zone is full, the surviving objects will be copied to the Survivor area (one of two), and when the survivor area is full, the surviving objects of this area will be copied to another survivor area, when the survivor is full, Objects that are copied from the first survivor area and that are still alive will be duplicated in the old age zone (tenured. It should be noted that the two areas of the survivor are symmetrical and have no relationship, so the same area may exist at the same time from Eden copied objects, and from the previous survivor copied objects, and copied to the old quarter only from the first survivor to come over the object. Moreover, there is always an empty survivor area.

2.Tenured (Old Generation)

Older generations store objects that survive from younger generations. In general, older generations are storing long-term objects.

3.Perm (Durable)

Used to store static files, now Java classes, methods, and so on. The persistence generation has no significant impact on garbage collection, but some applications may dynamically generate or invoke some classes, such as Hibernate, at which point a large, persistent generation space is required to store the new class in these runs. The persistent generation size is set by-xx:maxpermsize=.

For example, when an object is generated in a program, the normal object allocates space in the younger generation, and if the object is too large, it may be generated directly in the old generation (it is observed that each time a program is run, a 10 trillion space is generated for sending and receiving messages, and this memory is allocated directly to the old generation). The young generation will initiate a memory recovery when the space is allocated, most of the memory will be recycled, a portion of the surviving memory will be copied to the survivor from zone, and after multiple collections, if the From zone memory is also allocated, then a memory recycle will occur and the remaining objects will be copied to the to area. When the to area is full, a memory recycle occurs and the surviving objects are copied to the old quarter.

Usually we say that the JVM memory recycle is always referred to as heap memory recycling, indeed only the content in the heap is dynamically requested to allocate, so the young and old generations of the above objects refer to the JVM's heap space, and the persistent generation is the previously mentioned Methodarea, not the heap.

Some suggestions on JVM memory management

1. Manually set the generated garbage object, the intermediate object to null, and speed up the memory reclamation.

2. Object pooling technology If the resulting object is a reusable object, but the attributes are not the same, you can consider using an object pool to produce fewer objects. If a free object is removed from the object pool, no new objects are generated, which greatly increases the object's reuse rate.

3, JVM tuning by configuring the parameters of the JVM to increase the speed of garbage collection, if there is no memory leak and the above two methods can not guarantee the JVM memory recovery, you may consider the JVM tuning method to solve, but must go through the physical machine long-term testing, because different parameters may cause different effects. such as-XNOCLASSGC parameters.

Java base-JVM Memory reclamation