Java 8 的默认垃圾收集器

Question

What is the default garbage collector for Java 8?

When I check the JMX Beans, they reveal it to be the parallel collector for the new generation and the old serial collector for the old generation.

Answer 1

Default garbage collectors:

• Java 7 - Parallel GC
• Java 8 - Parallel GC
• Java 9 - G1 GC
• Java 10 - G1 GC

Answer 2

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.

Answer 3

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 

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

Answer 4

从 Java 9 到 Java 15，G1GC 是默认设置，即使在 Java 15 上也是默认的，它增加了两个新一代垃圾收集器，甲骨文开发的 ZGC 和红帽实现，即雪兰多，两者都在 2020 年 9 月的 Java 15 中投入生产。