《深入理解Java虚拟机》之长期存活的对象将进入老年代测试结果（-XX:MaxTenuringThreshold=15未生效）

水深无声 2022-05-26 12:29 77阅读 0赞

在阅读《深入理解Java虚拟机》中长期存活对象进入年老代这部分内容时，将书中代码运行了一遍，结果如下（运行环境：jdk1.8.0\_172）：

①当-XX:MaxTenuringThreshold=1时，运行结果与书中描述一致，allocation1对象在第二次GC发生时进入老年代。

\[GC (Allocation Failure) \[DefNew  
Desired survivor size 524288 bytes, **new threshold 1 (max 1)**  
\- age 1: 790240 bytes, 790240 total  
: 5335K->771K(9216K), 0.0031656 secs\] 5335K->4867K(19456K), 0.0032127 secs\] \[Times: user=0.02 sys=0.00, real=0.00 secs\]  
\[GC (Allocation Failure) \[DefNew  
Desired survivor size 524288 bytes, **new threshold 1 (max 1)**  
: 4867K->0K(9216K), 0.0008847 secs\] 8963K->4866K(19456K), 0.0009065 secs\] \[Times: user=0.00 sys=0.00, real=0.00 secs\]  
Heap  
def new generation total 9216K, used 4178K \[0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)  
eden space 8192K, 51% used \[0x00000000fec00000, 0x00000000ff014930, 0x00000000ff400000)  
**from space 1024K, 0%** used \[0x00000000ff400000, 0x00000000ff400000, 0x00000000ff500000)  
to space 1024K, 0% used \[0x00000000ff500000, 0x00000000ff500000, 0x00000000ff600000)  
tenured generation total 10240K, used 4866K \[0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)  
the space 10240K, 47% used \[0x00000000ff600000, 0x00000000ffac0b70, 0x00000000ffac0c00, 0x0000000100000000)  
Metaspace used 2620K, capacity 4486K, committed 4864K, reserved 1056768K  
class space used 280K, capacity 386K, committed 512K, reserved 1048576K

②当-XX:MaxTenuringThreshold=15时，运行结果与书中描述出现了偏差，allocation1对象在第二次GC发生时同样进入了老年代。

\[GC (Allocation Failure) \[DefNew  
Desired survivor size 524288 bytes, **new threshold 1 (max 15)**  
\- age 1: 790240 bytes, 790240 total  
: 5335K->771K(9216K), 0.0028299 secs\] 5335K->4867K(19456K), 0.0028776 secs\] \[Times: user=0.00 sys=0.00, real=0.00 secs\]  
\[GC (Allocation Failure) \[DefNew  
Desired survivor size 524288 bytes, **new threshold 15 (max 15)**  
: 4867K->0K(9216K), 0.0008606 secs\] 8963K->4866K(19456K), 0.0008757 secs\] \[Times: user=0.00 sys=0.00, real=0.00 secs\]  
Heap  
def new generation total 9216K, used 4178K \[0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)  
eden space 8192K, 51% used \[0x00000000fec00000, 0x00000000ff014930, 0x00000000ff400000)  
**from space 1024K, 0%** used \[0x00000000ff400000, 0x00000000ff400000, 0x00000000ff500000)  
to space 1024K, 0% used \[0x00000000ff500000, 0x00000000ff500000, 0x00000000ff600000)  
tenured generation total 10240K, used 4866K \[0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)  
the space 10240K, 47% used \[0x00000000ff600000, 0x00000000ffac0b70, 0x00000000ffac0c00, 0x0000000100000000)  
Metaspace used 2620K, capacity 4486K, committed 4864K, reserved 1056768K  
class space used 280K, capacity 386K, committed 512K, reserved 1048576K

上面运行结果中new threshold 1 (max 15)表示虚拟机自己计算的阈值为1 ，最大15。书中在-XX:MaxTenuringThreshold=15时，上面第一处标红的地方是new threshold 15 （max 15）。当在jdk 1.6.0\_37环境下运行时，会看到和书中一致的结果。所以猜测可能是最新版的虚拟机做了改动。