1、先来看看一个成功的按照预想进行了一次ygc的例子
/** * ygc测试 * -Xms10m -Xmx10m -Xmn5m -XX:+UseParallelGC -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -Xloggc:gc.log 设置10m堆大小，年轻代和老年代各分5m，年轻代里伊甸区4m、两个幸存者区都是0.5m * */ public class TestYoungGC2 { private static int _1MB = 1024*1024; public static void main(String[] args) { List cache = new ArrayList<byte[]>(); //只能循环三次新增3M对象、到第4次就会发生ygc，因为eden本身不是完全4M可用的 for (int i = 0; i < 4; i++){ System.out.println("循环" + (i+1) + "开始"); cache.add(new byte[_1MB]); cache.remove(0); System.out.println("循环" + (i+1) + "结束"); } System.out.println("此时ygc回收了3M垃圾剩余1M对象、但这1M对象也失去了引用，下一次ygc将被回收"); cache.add(new byte[2*_1MB]); System.out.println("此时又新分配2M对象到eden, 最后新生代3M+, 老年代接近0M"); } }
输出：
循环1开始
循环1结束
循环2开始
循环2结束
循环3开始
循环3结束
循环4开始
0.158: [GC (Allocation Failure) [PSYoungGen: 4046K->496K(4608K)] 4046K->644K(9728K), 0.0009270 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
循环4结束
此时ygc回收了3M垃圾剩余1M对象、但这1M对象也失去了引用，下一次ygc将被回收
此时又新分配2M对象到eden, 最后新生代3M+, 老年代接近0M
Heap
PSYoungGen total 4608K, used 3688K [0x00000000ffb00000, 0x0000000100000000, 0x0000000100000000)
eden space 4096K, 77% used [0x00000000ffb00000,0x00000000ffe1e368,0x00000000fff00000)
from space 512K, 96% used [0x00000000fff00000,0x00000000fff7c020,0x00000000fff80000)
to space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)
ParOldGen total 5120K, used 148K [0x00000000ff600000, 0x00000000ffb00000, 0x00000000ffb00000)
object space 5120K, 2% used [0x00000000ff600000,0x00000000ff625010,0x00000000ffb00000)
Metaspace used 2579K, capacity 4486K, committed 4864K, reserved 1056768K
class space used 286K, capacity 386K, committed 512K, reserved 1048576K
2、再看一个使用byte[]数组模拟ygc的例子，比较特殊
-XX:NewSize=5m -XX:MaxNewSize=5m -XX:InitialHeapSize=10m -XX:MaxHeapSize=10m -XX:SurvivorRatio=8 -XX:+PrintGCDetails -XX:+PrintGCTimeStamps
public class TestYoungGC { public static void main(String[] args) { byte[] array1 = new byte[1024*1024]; array1 = new byte[1024*1024]; array1 = new byte[1024*1024]; array1 = null; //上面3M对象变成垃圾对象，可以被回收 byte[] array2 = new byte[2*1024*1024]; //array2在eden分配不下，应该触发ygc } }
运行结果：
Heap
PSYoungGen total 4608K, used 4096K [0x00000000ffb00000, 0x0000000100000000, 0x0000000100000000)
eden space 4096K, 100% used [0x00000000ffb00000,0x00000000fff00000,0x00000000fff00000)
from space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)
to space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)
ParOldGen total 5120K, used 2048K [0x00000000ff600000, 0x00000000ffb00000, 0x00000000ffb00000)
object space 5120K, 40% used [0x00000000ff600000,0x00000000ff800010,0x00000000ffb00000)
Metaspace used 2577K, capacity 4486K, committed 4864K, reserved 1056768K
class space used 286K, capacity 386K, committed 512K, reserved 1048576K
意料之外的事情发生了：在向eden新分了3M对象之后，array2的2M对象并没有按照剧本来触发ygc回收先前的3M垃圾，然后再把这2M分到eden。
从结果来看是这2M被直接分到老年代了，而eden因为刚好没有满，所以3M垃圾到最后也没有ygc回收掉。
这种现象在 http://www.reins.altervista.org/java/gc1.4.2_faq.html 这篇文章有很好的解释：
有两种情况对象是可能直接进入老年代，而不是尝试在新生代分配：
1、你要分配的对象是一个大数组、且不包含其他对象的引用。
2、你手工设置了-XX:PretenureSizeThreshold参数来指定直接进入老年代的对象的大小。
所以为了导演出原来预期的结果，我们分别模拟下上面的两种方法
1、首先，应该是array2太大了，先把它弄小点，最后试验出来：array2 设置小于等于2*1024*1024 -24触发ygc，大于等于2*1024*1024 -23不触发ygc，直接进入了老年代。
所以代码修改如下：
public class TestYoungGC { public static void main(String[] args) { byte[] array1 = new byte[1024*1024]; array1 = new byte[1024*1024]; array1 = new byte[1024*1024]; array1 = null; //上面3M对象变成垃圾对象，可以被回收 byte[] array2 = new byte[2*1024*1024 - 24]; //array2在eden分配不下，应该触发ygc } }
输出：
0.155: [GC (Allocation Failure) [PSYoungGen: 4046K->496K(4608K)] 4046K->652K(9728K), 0.0011635 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
PSYoungGen total 4608K, used 2585K [0x00000000ffb00000, 0x0000000100000000, 0x0000000100000000)
eden space 4096K, 51% used [0x00000000ffb00000,0x00000000ffd0a540,0x00000000fff00000)
from space 512K, 96% used [0x00000000fff00000,0x00000000fff7c040,0x00000000fff80000)
to space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)
ParOldGen total 5120K, used 156K [0x00000000ff600000, 0x00000000ffb00000, 0x00000000ffb00000)
object space 5120K, 3% used [0x00000000ff600000,0x00000000ff627010,0x00000000ffb00000)
Metaspace used 2577K, capacity 4486K, committed 4864K, reserved 1056768K
class space used 286K, capacity 386K, committed 512K, reserved 1048576K
终于按照剧本来了！
2、第二个方法，还是byte[] array2 = new byte[2*1024*1024]，然后设置-XX:PretenureSizeThreshold=10m ，指定超过10M的对象才能直接进入老年代
结果：
Heap
PSYoungGen total 4608K, used 4096K [0x00000000ffb00000, 0x0000000100000000, 0x0000000100000000)
eden space 4096K, 100% used [0x00000000ffb00000,0x00000000fff00000,0x00000000fff00000)
from space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)
to space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)
ParOldGen total 5120K, used 2048K [0x00000000ff600000, 0x00000000ffb00000, 0x00000000ffb00000)
object space 5120K, 40% used [0x00000000ff600000,0x00000000ff800010,0x00000000ffb00000)
Metaspace used 2577K, capacity 4486K, committed 4864K, reserved 1056768K
class space used 286K, capacity 386K, committed 512K, reserved 1048576K
又不按剧本走了！ 查阅网上资料，PretenureSizeThreshold这个参数只对Serial和ParNew两款收集器有效，笔者电脑在jdk8环境下默认的垃圾收集器是parallel，不认这个参数。
所以调整一下收集器，再次修改jvm启动参数：
-XX:NewSize=5m -XX:MaxNewSize=5m -XX:InitialHeapSize=10m -XX:MaxHeapSize=10m -XX:SurvivorRatio=8 -XX:PretenureSizeThreshold=10m -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:PretenureSizeThreshold=10m -XX:+UseParNewGC
运行：
0.158: [GC (Allocation Failure) 0.158: [ParNew: 4046K->512K(4608K), 0.0012050 secs] 4046K->632K(9728K), 0.0012951 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
par new generation total 4608K, used 2601K [0x00000000ff600000, 0x00000000ffb00000, 0x00000000ffb00000)
eden space 4096K, 51% used [0x00000000ff600000, 0x00000000ff80a558, 0x00000000ffa00000)
from space 512K, 100% used [0x00000000ffa80000, 0x00000000ffb00000, 0x00000000ffb00000)
to space 512K, 0% used [0x00000000ffa00000, 0x00000000ffa00000, 0x00000000ffa80000)
tenured generation total 5120K, used 120K [0x00000000ffb00000, 0x0000000100000000, 0x0000000100000000)
the space 5120K, 2% used [0x00000000ffb00000, 0x00000000ffb1e000, 0x00000000ffb1e000, 0x0000000100000000)
Metaspace used 2577K, capacity 4486K, committed 4864K, reserved 1056768K
class space used 286K, capacity 386K, committed 512K, reserved 1048576K
Java HotSpot(TM) 64-Bit Server VM warning: Using the ParNew young collector with the Serial old collector is deprecated and will likely be removed in a future release
终于如愿以偿按照剧本来了，最后变有个warning，意思是仅指定parNew作为新生代收集器的话、jvm默认给搭配的是serial old老年代收集器。要消除warning，手工指定一下别的老年代收集器即可，比如-XX:+UseConcMarkSweepGC
参考资料：
http://www.reins.altervista.org/java/gc1.4.2_faq.html
https://www.spirithy.com/2019/04/01/trigger-jvm-full-gc-young-gc/