这6M的内容转入老年代中。然后再来一个4M的，由于此时Minor GC之后新生代只剩下了194K了，够分配了，所以4M顺利进入新生代。
2、Server模式下，前面都一样，但是在GC的时候有一点区别。在GC前还会进行一次判断，如果要分配的内存>=Eden区大小的一半，那么会直接把要分配的内存放入老年代中。要分配4M，Eden区8M，刚好一半，而且老年代10M，够分配，所以4M就直接进入老年代去了。为了验证一下结论，我们把3个2M之后分配的4M改为3M看一下
public class EdenAllocationTest
{
private static final int _1MB = 1024 * 1024;
public static void main(String[] args)
{
byte[] allocation1 = new byte[2 * _1MB];
byte[] allocation2 = new byte[2 * _1MB];
byte[] allocation3 = new byte[2 * _1MB];
byte[] allocation4 = new byte[3 * _1MB];
}
}
运行结果为
[GC [PSYoungGen: 6487K->352K(9216K)] 6487K->6496K(19456K), 0.0035661 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[Full GC [PSYoungGen: 352K->0K(9216K)] [PSOldGen: 6144K->6338K(10240K)] 6496K->6338K(19456K) [PSPermGen: 2941K->2941K(21248K)], 0.0035258 secs] [Times: user=0.02 sys=0.00, real=0.00 secs]
Heap
PSYoungGen total 9216K, used 3236K [0x000000000af40000, 0x000000000b940000, 0x000000000b940000)
eden space 8192K, 39% used [0x000000000af40000,0x000000000b269018,0x000000000b740000)
from space 1024K, 0% used [0x000000000b740000,0x000000000b740000,0x000000000b840000)
to space 1024K, 0% used [0x000000000b840000,0x000000000b840000,0x000000000b940000)
PSOldGen total 10240K, used 6338K [0x000000000a540000, 0x000000000af40000, 0x000000000af40000)
object space 10240K, 61% used [0x000000000a540000,0x000000000ab70858,0x000000000af40000)
PSPermGen total 21248K, used 2982K [0x0000000005140000, 0x0000000006600000, 0x000000000a540000)
object space 21248K, 14% used [0x0000000005140000,0x0000000005429890,0x0000000006600000)
看到3M在新生代中，6M通过分配担保机制进入老年代了。
大对象直接进入老年代
虚拟机参数为“-XX:+PrintGCDetails -Xms20M -Xmx20M -Xmn10M -XX:SurvivorRatio=8 -XX:PretenureSizeThreshold=3145728”，最后那个参数表示大于这个设置值的对象直接在老年代中分配，这样做的目的是为了避免在Eden区和两个Survivor区之间发生大量的内存复制。测试代码为
public class OldTest
{
private static final int _1MB = 1024 * 1024;
public static void main(String[] args)
{
byte[] allocation = new byte[4 * _1MB];
}
}
Client模式下
Heap
def new generation total 9216K, used 507K [0x0000000005140000, 0x0000000005b40000, 0x0000000005b40000)
eden space 8192K, 6% used [0x0000000005140000, 0x00000000051bef28, 0x0000000005940000)
from space 1024K, 0% used [0x0000000005940000, 0x0000000005940000, 0x0000000005a40000)
to space 1024K, 0% used [0x0000000005a40000, 0x0000000005a40000, 0x0000000005b40000)
tenured generation total 10240K, used 4096K [0x0000000005b40000, 0x0000000006540000, 0x0000000006540000)
the space 10240K, 40% used [0x0000000005b40000, 0x0000000005f40018, 0x0000000005f40200, 0x0000000006540000)
compacting perm gen total 21248K, used 2972K [0x0000000006540000, 0x0000000007a00000, 0x000000000b940000)
the space 21248K, 13% used [0x0000000006540000, 0x00000000068272a0, 0x0000000006827400, 0x0000000007a00000)
No shared spaces configured.
Server模式下
Heap
PSYoungGen total 9216K, used 4603K [0x000000000afc0000, 0x000000000b9c0000, 0x000000000b9c0000)
eden space 8192K, 56% used [0x000000000afc0000,0x000000000b43ef40,0x000000000b7c0000)
from space 1024K, 0% used [0x000000000b8c0000,0x000000000b8c0000,0x000000000b9c0000)
to space 1024K, 0% used [0x000000000b7c0000,0x000000000b7c0000,0x000000000b8c0000)
PSOldGen total 10240K,