Java 7 (JDK 7) garbage collection and documentation on G1

Question

Java 7 has been out for a while now, but I cannot find any good resources on the configuration of the garbage collectors, specifically the new G1 collector.

My questions:

1. Is G1 the default collector in Java 7 and if not how do I activate G1?
2. What optional settings does g1 have in Java7?
3. Were there any changes made to other collectors like cms or the parallel collector in Java 7?
4. Where can I find good documentation on garbage collection in Java 7?

Answer 1

The G1 garbage collector is not the default in my installation of Java, version 1.7.0_01. You can see for yourself by using with some extra command line options:

> java -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -version -XX:InitialHeapSize=132304640 -XX:MaxHeapSize=2116874240 -XX:ParallelGCThreads=4 -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -XX:+UseCompressedOops -XX:-UseLargePagesIndividualAllocation -XX:+UseParallelGC java version "1.7.0_01" Java(TM) SE Runtime Environment (build 1.7.0_01-b08) Java HotSpot(TM) 64-Bit Server VM (build 21.1-b02, mixed mode) Heap  PSYoungGen      total 37696K, used 1293K [0x00000007d5eb0000, 0x00000007d88c0000, 0x0000000800000000)   eden space 32320K, 4% used [0x00000007d5eb0000,0x00000007d5ff3408,0x00000007d7e40000)   from space 5376K, 0% used [0x00000007d8380000,0x00000007d8380000,0x00000007d88c0000)   to   space 5376K, 0% used [0x00000007d7e40000,0x00000007d7e40000,0x00000007d8380000)  PSOldGen        total 86144K, used 0K [0x0000000781c00000, 0x0000000787020000, 0x00000007d5eb0000)   object space 86144K, 0% used [0x0000000781c00000,0x0000000781c00000,0x0000000787020000)  PSPermGen       total 21248K, used 2032K [0x000000077ca00000, 0x000000077dec0000, 0x0000000781c00000)   object space 21248K, 9% used [0x000000077ca00000,0x000000077cbfc288,0x000000077dec0000) 

You don't need to enable experimental options to turn on the G1 collector any more, though:

> java -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -XX:+UseG1GC -version -XX:InitialHeapSize=132304640 -XX:MaxHeapSize=2116874240 -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -XX:+UseCompressedOops -XX:+UseG1GC -XX:-UseLargePagesIndividualAllocation java version "1.7.0_01" Java(TM) SE Runtime Environment (build 1.7.0_01-b08) Java HotSpot(TM) 64-Bit Server VM (build 21.1-b02, mixed mode) Heap  garbage-first heap   total 130048K, used 0K [0x000000077ca00000, 0x0000000784900000, 0x00000007fae00000)   region size 1024K, 1 young (1024K), 0 survivors (0K)  compacting perm gen  total 20480K, used 2032K [0x00000007fae00000, 0x00000007fc200000, 0x0000000800000000)    the space 20480K,   9% used [0x00000007fae00000, 0x00000007faffc288, 0x00000007faffc400, 0x00000007fc200000) No shared spaces configured. 

I don't know where you can find any good documentation.

Answer 2

Oracle finally made G1 official in Java 7 U4: http://www.oracle.com/technetwork/java/javase/7u4-relnotes-1575007.html

Description: http://docs.oracle.com/javase/7/docs/technotes/guides/vm/G1.html

Command line options: http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html#G1Options

Still, I do not think it is the default collector in Java 7. For servers the default is the Parallel Collector as in Java 6.