Java SE 5:根据垃圾收集器工效学[Oracle]:

初始堆大小: 大于机器物理内存的1/64 合理的最低。在J2SE 5.0之前， 默认的初始堆大小为 合理的最小值，由 平台。你可以重写这个 默认使用-Xms命令行 选择。 最大堆大小: 小于物理内存的1/4或1GB。在J2SE 5.0之前， 默认最大堆大小为64MB。 您可以使用 -Xmx命令行选项。

更新:

正如Tom Anderson在他的评论中所指出的，以上是针对服务器级机器的。来自5.0 JavaTM虚拟机中的人机工程学:

In the J2SE platform version 5.0 a class of machine referred to as a server-class machine has been defined as a machine with 2 or more physical processors 2 or more Gbytes of physical memory with the exception of 32 bit platforms running a version of the Windows operating system. On all other platforms the default values are the same as the default values for version 1.4.2. In the J2SE platform version 1.4.2 by default the following selections were made initial heap size of 4 Mbyte maximum heap size of 64 Mbyte

Java SE 5:根据垃圾收集器工效学[Oracle]:

初始堆大小: 大于机器物理内存的1/64 合理的最低。在J2SE 5.0之前， 默认的初始堆大小为 合理的最小值，由 平台。你可以重写这个 默认使用-Xms命令行 选择。 最大堆大小: 小于物理内存的1/4或1GB。在J2SE 5.0之前， 默认最大堆大小为64MB。 您可以使用 -Xmx命令行选项。

更新:

正如Tom Anderson在他的评论中所指出的，以上是针对服务器级机器的。来自5.0 JavaTM虚拟机中的人机工程学:

In the J2SE platform version 5.0 a class of machine referred to as a server-class machine has been defined as a machine with 2 or more physical processors 2 or more Gbytes of physical memory with the exception of 32 bit platforms running a version of the Windows operating system. On all other platforms the default values are the same as the default values for version 1.4.2. In the J2SE platform version 1.4.2 by default the following selections were made initial heap size of 4 Mbyte maximum heap size of 64 Mbyte

在运行时根据系统配置选择默认值

看一下文档页

默认堆大小 除非在命令行上指定了初始和最大堆大小，否则它们将根据计算机上的内存量计算。

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). Server JVM Default Initial and Maximum Heap Sizes: 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

哪些系统配置设置会影响默认值?

您可以使用-Xms(初始堆大小)和-Xmx(最大堆大小)标志指定初始和最大堆大小。如果您知道应用程序需要多少堆才能正常工作，那么可以将-Xms和-Xmx设置为相同的值

终于!

从Java 8u191开始，你现在有以下选项:

-XX:InitialRAMPercentage
-XX:MaxRAMPercentage
-XX:MinRAMPercentage

可用于按可用物理RAM的百分比来确定堆的大小。(这与安装的RAM比内核使用的少)。

有关更多信息，请参阅Java8 u191的发布说明。请注意，这些选项是在Docker标题下提到的，但实际上无论您是在Docker环境中还是在传统环境中，它们都适用。

MaxRAMPercentage的默认值是25%。这是极端保守的。

My own rule: If your host is more or less dedicated to running the given java application, then you can without problems increase dramatically. If you are on Linux, only running standard daemons and have installed RAM from somewhere around 1 Gb and up then I wouldn't hesitate to use 75% for the JVM's heap. Again, remember that this is 75% of the RAM available, not the RAM installed. What is left is the other user land processes that may be running on the host and the other types of memory that the JVM needs (eg for stack). All together, this will typically fit nicely in the 25% that is left. Obviously, with even more installed RAM the 75% is a safer and safer bet. (I wish the JDK folks had implemented an option where you could specify a ladder)

设置MaxRAMPercentage选项如下所示:

java -XX:MaxRAMPercentage=75.0  ....

请注意，这些百分比值是“double”类型，因此必须使用小数点指定它们。如果使用“75”而不是“75.0”，就会得到一个有点奇怪的错误。

欧内斯托是对的。根据他发布的链接[1]:

Updated Client JVM heap configuration In the Client JVM... The default maximum heap size is half of the physical memory up to a physical memory size of 192 megabytes and otherwise one fourth of the physical memory up to a physical memory size of 1 gigabyte. For example, if your machine has 128 megabytes of physical memory, then the maximum heap size is 64 megabytes, and greater than or equal to 1 gigabyte of physical memory results in a maximum heap size of 256 megabytes. The maximum heap size is not actually used by the JVM unless your program creates enough objects to require it. A much smaller amount, termed the initial heap size, is allocated during JVM initialization. ... ... Server JVM heap configuration ergonomics are now the same as the Client, except that the default maximum heap size for 32-bit JVMs is 1 gigabyte, corresponding to a physical memory size of 4 gigabytes, and for 64-bit JVMs is 32 gigabytes, corresponding to a physical memory size of 128 gigabytes.

[1] http://www.oracle.com/technetwork/java/javase/6u18-142093.html

在Windows上，可以使用以下命令找出运行应用程序的系统上的默认值。

java -XX:+PrintFlagsFinal -version | findstr HeapSize

寻找选项MaxHeapSize(用于-Xmx)和InitialHeapSize(用于-Xms)。

在Unix/Linux系统上，您可以这样做

java -XX:+PrintFlagsFinal -version | grep HeapSize

我认为结果输出是以字节为单位的。