什么都没做就占用了内存100m

Question

上传服务是用jetty起的，可能一些大神没研究过，我估计tomcat起也一样
900多喝400多就不应该了，我感觉除以100，才符合我的预期啊，难道webser加载了那么多class吗？

Answer 1

jvm虚拟机每个内存区域都有默认初始大小的。下面截取oracle java官方文档。

Default Heap Size

Unless the initial and maximum heap sizes are specified on the command line, they are calculated based on the amount of memory on the machine.

Client JVM Default Initial and Maximum Heap Sizes

The default maximum heap size is half of the physical memory up to a physical memory size of 192 megabytes (MB) and otherwise one fourth of the physical memory up to a physical memory size of 1 gigabyte (GB).

For example, if your computer has 128 MB of physical memory, then the maximum heap size is 64 MB, and greater than or equal to 1 GB of physical memory results in a maximum heap size of 256 MB.

The maximum heap size is not actually used by the JVM unless your program creates enough objects to require it. A much smaller amount, called the initial heap size, is allocated during JVM initialization. This amount is at least 8 MB and otherwise 1/64th of physical memory up to a physical memory size of 1 GB.

The maximum amount of space allocated to the young generation is one third of the total heap size.

Server JVM Default Initial and Maximum Heap Sizes

The default initial and maximum heap sizes work similarly on the server JVM as it does on the client JVM, except that the default values can go higher. On 32-bit JVMs, the default maximum heap size can be up to 1 GB if there is 4 GB or more of physical memory. On 64-bit JVMs, the default maximum heap size can be up to 32 GB if there is 128 GB or more of physical memory. You can always set a higher or lower initial and maximum heap by specifying those values directly; see the next section.

如果你的应用确实不需要那么多的内存，设置一下就可以了。

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