web123456

Java's GC (Garbage Collection, Garbage Collection)

Java's GC (Garbage Collection) mechanism is JavaVirtual MachineAn important feature in (JVM) is used to automatically manage objects that are no longer used in heap memory and free up the memory space it occupies to avoidMemory leakand overflow. The following is a detailed analysis of the Java GC mechanism:

1. The basic principles of GC

GC realizes memory recycling and release by marking and recycling invalid objects. The main processes include:

  1. Tags for objects: GC first marks out all active objects, that is, objects that are still referenced or accessible. It starts with a set of root objects (GC Roots), gradually traverses the object graph, marking accessible objects as active objects, and unmarked objects are considered invalid.

  2. Garbage recycling: After the marking is completed, GC will recycle unmarked objects. Specific recycling algorithms can be different. Common algorithms include mark-copy algorithms, mark-clearing algorithms, mark-tidy algorithms, mark-tidy algorithms, etc.

  3. Memory compression and organization(Optional): SomeGarbage recycling algorithmAfter reclaiming the object, memory fragmentation may occur. In order to optimize memory usage, GC may perform memory compression and sorting operations, so that allocated objects are continuously stored in memory, reducing the impact of memory fragmentation.

2. GC trigger timing

The triggering timing of GC is uncertain, mainly determined by the memory usage of the JVM. When the free space in heap memory is not enough to meet the allocation needs of new objects, the JVM will trigger GC to recycle garbage objects and free up memory space.

3. Types of GC

Java Virtual MachineThe GCs in them can be divided into many types, mainly based on the recycled memory area and the algorithms used:

  1. Young GC（Minor GC）：

    • It mainly carries out garbage collection for the new generation (Young Generation).
    • The goal is to recycle garbage objects in the new generation as soon as possible to make more space for the allocation of new objects.
    • Because the life cycle of objects in the Cenozoic generation is short, the frequency of Young GC is usually relatively high, but the number of objects collected each time is relatively small, and the overhead of GC is also small.

  2. Full GC（Major GC）：

    • It is a process of garbage collection of the entire heap memory (including the new generation and the old generation).
    • The triggering of Full GC can cause a longer pause because it requires scanning and recycling the entire heap memory, marking and clearing a large number of objects.
    • Full GC has relatively large overhead, and the corresponding STW (Stop-The-World) time is longer, so frequent triggers should be avoided as much as possible.

4. GC algorithm

Commonly used garbage collection algorithms in Java virtual machines include:

  1. Mark-clearing algorithm (Mark-and-Sweep)：

    • It is divided into two stages: marking and clearing.
    • First, all active objects are marked out through accessibility analysis, and then all unlabeled objects are cleaned out during the cleaning phase.
    • The advantage is that it is simple and direct, but the disadvantage is that it will generate a large amount of memory fragmentation after marks are cleared.

  2. Mark-Copying algorithm：

    • Divide the heap memory into two areas, using only one of them at a time.
    • When this area is full, copy the surviving object to another area and clear the current area.
    • The advantage is that memory fragmentation is avoided, but the disadvantage is that only half of the actual available memory is.

  3. Mark-Compact：

    • Similar to the mark-clearing algorithm, but during the clearing phase, all surviving objects are compressed to one end of the heap.
    • The advantage is that memory fragmentation is eliminated, but the disadvantage is that execution efficiency is relatively low.

  4. Generational Collection：

    • The heap memory is divided into the new generation and the old generation, and different recycling strategies are adopted according to the survival time of the object.
    • The life cycle of the new generation objects is short, and the replication algorithm is used to quickly recycle; the life cycle of the old generation objects is long, and the label-clear or label-organization algorithm is used to recycle.

5. GC optimization

When developing Java applications, placing GC parameters reasonably and adjusting garbage collection strategies to improve the programperformanceand stability is crucial. Common tuning strategies include:

  • Resize the heap (such as the initial heap size-Xmsand maximum heap size-Xmx）。
  • Adjust the size of the Cenozoic Generation (such as the Cenozoic Generation-Xmnand the ratio of the new generation to the old generation-XX:NewRatio）。
  • Choose the right garbage collector (such as Serial GC, Parallel Scavenge GC, CMS GC, G1 GC, etc.).
  • Enable garbage collection logs (e.g.-XX:+PrintGCDetailsand-Xloggc) in order to analyze and tune GC strategies.

Through the above measures, the frequency and pause time of GC can be effectively controlled, and the performance and stability of Java applications can be improved.