一个很简单的GC测试，但是没有打印出预期的效果。
JDK1.8.0_131,JVM参数如下:

-Xmx50m -Xms50m -Xmn8m -XX:SurvivorRatio=2 -XX:+PrintGCDetails -XX:+PrintGC

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public static void main(String[] args) { Object[] list = new Object[10]; for(int i=0; i<10 ; i++) { System.out.println(">>>>>>>>>>" + i); list[i] = new byte[3*1024*1024]; } }
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console打印如下:

>>>>>>>>>>0 >>>>>>>>>>1 >>>>>>>>>>2 >>>>>>>>>>3 >>>>>>>>>>4 >>>>>>>>>>5 >>>>>>>>>>6 >>>>>>>>>>7 >>>>>>>>>>8 >>>>>>>>>>9 Heap PSYoungGen total 6144K, used 2139K [0x00000000ff800000, 0x0000000100000000, 0x0000000100000000) eden space 4096K, 52% used [0x00000000ff800000,0x00000000ffa16fa0,0x00000000ffc00000) from space 2048K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x0000000100000000) to space 2048K, 0% used [0x00000000ffc00000,0x00000000ffc00000,0x00000000ffe00000) ParOldGen total 43008K, used 30720K [0x00000000fce00000, 0x00000000ff800000, 0x00000000ff800000) object space 43008K, 71% used [0x00000000fce00000,0x00000000fec000a0,0x00000000ff800000) Metaspace used 3498K, capacity 4498K, committed 4864K, reserved 1056768K class space used 387K, capacity 390K, committed 512K, reserved 1048576K
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虽然最终看到对象进入了老年代，但是Minor GC的过程都没有打印出来，如果将new byte[310241024] 改为 new byte[110241024]，即如下如所示:

public static void main(String[] args) { Object[] list = new Object[10]; for(int i=0; i<10 ; i++) { System.out.println(">>>>>>>>>>" + i); list[i] = new byte[1*1024*1024]; } }
console打印如下:

>>>>>>>>>>0 >>>>>>>>>>1 [GC (Allocation Failure) [PSYoungGen: 3081K->1896K(6144K)] 3081K->1904K(49152K), 0.0014582 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] >>>>>>>>>>2 >>>>>>>>>>3 >>>>>>>>>>4 [GC (Allocation Failure) [PSYoungGen: 5087K->1864K(6144K)] 5095K->4944K(49152K), 0.0017647 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] >>>>>>>>>>5 >>>>>>>>>>6 >>>>>>>>>>7 [GC (Allocation Failure) [PSYoungGen: 5124K->1784K(6144K)] 8204K->7936K(49152K), 0.0012156 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] >>>>>>>>>>8 >>>>>>>>>>9 Heap PSYoungGen total 6144K, used 5009K [0x00000000ff800000, 0x0000000100000000, 0x0000000100000000) eden space 4096K, 78% used [0x00000000ff800000,0x00000000ffb26620,0x00000000ffc00000) from space 2048K, 87% used [0x00000000ffc00000,0x00000000ffdbe040,0x00000000ffe00000) to space 2048K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x0000000100000000) ParOldGen total 43008K, used 6152K [0x00000000fce00000, 0x00000000ff800000, 0x00000000ff800000) object space 43008K, 14% used [0x00000000fce00000,0x00000000fd402060,0x00000000ff800000) Metaspace used 3495K, capacity 4498K, committed 4864K, reserved 1056768K class space used 387K, capacity 390K, committed 512K, reserved 1048576K
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可以看到有清楚的Minor GC的过程，这是为什么呢？是JVM对这种大对象做了优化以至于直接分配至老年代了吗？