Java8的默认垃圾收集器

Java8的默认垃圾收集器是什么？

当我检查 JMX Beans 时，他们发现它是新一代的并行收集器，而旧一代的串行收集器。

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Selecting the default garbage collector (among other things) is what's called the ergonomics process of the JVM. This process depends on the class of your machine.

  • For server-class machine, defined as a machine with 2 or more physical processors and 2 or more GB of physical memory (regardless of the platform), the default garbage collector is the parallel collector (also known as throughput collector).
  • For client-class machine, defined as a 32-bit platform on Windows or a single-processor machine, the default garbage collector is the serial collector.

Since practically all machines have 2 or more CPU, a machine is practically always considered server-class by the JVM. That's why you will find a lot of references considering the parallel collector to be the default garbage collector.

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Default garbage collectors:

  • Java 7 - Parallel GC
  • Java 8 - Parallel GC
  • Java 9 - G1 GC
  • Java 10 - G1 GC
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Java has four types of garbage collectors(Up to version 10),but after stable release of java 11 , it would be 5 types. These are:-

  1. Serial Garbage Collector- S GC
  2. Parallel Garbage Collector- P GC
  3. CMS Garbage Collector- CMS GC
  4. G1 Garbage Collector- G1 GC
  5. The Z Garbage Collector- ZGC

Default implementations of GC in java -

JVM GC 

 Java 7 - P GC
Java 8 - P GC
Java 9 - G1 GC
Java 10- G1 GC
Java 11- Z GC(I am not sure but it would be default GC of java 11)

More details for ZGC,please visit

http://openjdk.java.net/projects/zgc/

https://www.opsian.com/blog/javas-new-zgc-is-very-exciting/

Note: If you want to verify, which GC is currently being used by JVM,you can go for following command to show default GC:-

$ java -XX:+PrintCommandLineFlags -version

If you want to set GC according to your need, you can do this by following command. Here I am going to set G1 GC as default GC.

$ java -XX:+UseG1GC -XX:+PrintCommandLineFlags -version

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For more details , please visit

https://javapapers.com/java/types-of-java-garbage-collectors/

https://alvinalexander.com/java/java-jvm-how-show-which-garbage-collector-running

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Since Java 9 until Java 15 the G1GC is the default even on Java 15 which adds two new generation Garbage collectors the one that Oracle develop ZGC and a red hat implementation which is Shenandoah both are production ready in Java 15 september of 2020. ZGC is available on OPEN JDK https://wiki.openjdk.java.net/display/zgc/Main


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