• 永久代的大小是如何变化的？

  The permanent generation is used to hold reflective of the VM itself such as class objects and method objects. These reflective objects are allocated directly into the permanent generation, and it is sized independently from the other generations. Generally, sizing of this generation can be ignored because the default size is adequate. However, programs that load many classes may need a larger permanent generation.

• 如何知道永久代是否已满？

　　　　Starting in 1.4.2 -XX:+PrintGCDetails will print information about all parts of the heap collected at each garbage collection. For a full collection[Full GC [Tenured: 30437K->33739K(280576K), 0.7050569 secs] 106231K->33739K(362112K), [Perm : 2919K->2919K(16384K)], 0.7052334 secs]

this example shows that little was collected in the permanent generation (it went from 2919K used before the collection to 2919K used after the collection) and the current size of the permanent generation is 16384K.

• 如何增加永久代大小？

Use the command line option -XX:MaxPermSize=<desired size>

• 如何知道加载或卸载了哪些类？

Use the command line options -XX:+TraceClassloading and -XX:+TraceClassUnloading

• 是否在client vm中增加永久代大小?

This will always be a judgment call. In general increasing the size of a generation (and this applies not just to the permanent generation) can reduce the incidence of a wide variety of problems However, this may cause other processes to excessively page and/or garbage collect or throw out-of-memory exceptions.

There are two failure modes to consider.

When raising MaxPermSize, it is possible that previously well behaved programs that used to garbage collect to recover the permanent generation space will die by endless paging. For the permanent generation this usually only happens with the heavy interning of temporary strings.

The other failure mode is that address space must be reserved for the permanent generation and this will reduce that available for the rest of the heap (the maximum -Xmx may then be too large). This will cause programs configured to use all available space to fail at initialization.