java垃圾回收那点事（二）究竟有多少GC

java的gc回收的类型主要有几种 UseSerialGC，UseConcMarkSweepGC，UseParNewGC，UseParallelGC，UseParallelOldGC，UseG1GC，而这几个参数是如何搭配的，实际上只要看下面的代码就非常清楚

bool Arguments::check_gc_consistency() {
  bool status = true;
  // Ensure that the user has not selected conflicting sets
  // of collectors. [Note: this check is merely a user convenience;
  // collectors over-ride each other so that only a non-conflicting
  // set is selected; however what the user gets is not what they
  // may have expected from the combination they asked for. It's
  // better to reduce user confusion by not allowing them to
  // select conflicting combinations.
  uint i = 0;
  if (UseSerialGC)                       i++;
  if (UseConcMarkSweepGC || UseParNewGC) i++;
  if (UseParallelGC || UseParallelOldGC) i++;
  if (UseG1GC)                           i++;
  if (i > 1) {
    jio_fprintf(defaultStream::error_stream(),
                "Conflicting collector combinations in option list; "
                "please refer to the release notes for the combinations "
                "allowed\n");
    status = false;
  }

  return status;
}

我们把GC分成4种类型

1.  SerialGC 

参数-XX:+UseSerialGC

就是Young区和old区都使用serial 垃圾回收算法，

2.  ParallelGC 

参数-XX:+UseParallelGC

Young区：使用Parallel scavenge 回收算法

Old  区：可以使用单线程的或者Parallel 垃圾回收算法，由 -XX:+UseParallelOldGC 来控制

3.  CMS  

参数-XX:+UseConcMarkSweepGC

Young区：可以使用普通的或者parallel 垃圾回收算法，由参数 -XX:+UseParNewGC来控制

Old 区：只能使用Concurrent Mark Sweep 

4. G1 

参数：-XX:+UseG1GC

没有young/old区