XX 的默认值：MaxDirectMemorySize

Question

XX:MaxDirectMemorySize 的默认值是多少？

Answer 1

来自 sun.misc.VM，它是Runtime.getRuntime.maxMemory()，这就是用-Xmx配置的。例如如果您没有配置-XX:MaxDirectMemorySize并且配置-Xmx5g，则“默认”>MaxDirectMemorySize 也将为 5 Gb，应用程序的总堆+直接内存使用量可能会增长到 5 + 5 = 10 Gb。

Answer 2

来自 http://www.docjar.com/html/api/ sun/misc/VM.java.html

我明白了：

 163       // A user-settable upper limit on the maximum amount of allocatable direct
 164       // buffer memory.  This value may be changed during VM initialization if
 165       // "java" is launched with "-XX:MaxDirectMemorySize=<size>".
 166       //
 167       // The initial value of this field is arbitrary; during JRE initialization
 168       // it will be reset to the value specified on the command line, if any,
 169       // otherwise to Runtime.getRuntime.maxDirectMemory().
 170       //
 171       private static long directMemory = 64 * 1024 * 1024;

所以它似乎默认为 64 兆。

Answer 3

对于JDK8：

64MB最初是任意设置的，...

（来自：https://github.com/frohoff/jdk8u-dev-jdk/blob/master/src/share/classes/sun/misc/VM.java#L186 )

    // A user-settable upper limit on the maximum amount of allocatable direct
    // buffer memory.  This value may be changed during VM initialization if
    // "java" is launched with "-XX:MaxDirectMemorySize=<size>".
    //
    // The initial value of this field is arbitrary; during JRE initialization
    // it will be reset to the value specified on the command line, if any,
    // otherwise to Runtime.getRuntime().maxMemory().
    //
    private static long directMemory = 64 * 1024 * 1024;

...但是然后 directMemory 设置为 maxMemory() ~= Heapsize (如果未设置 maxDirectMemorySize-Parameter):

(来自: https://github.com/frohoff/jdk8u-dev-jdk/blob/master /src/share/classes/sun/misc/VM.java#L286 ）

  // Set the maximum amount of direct memory.  This value is controlled
  // by the vm option -XX:MaxDirectMemorySize=<size>.
  // The maximum amount of allocatable direct buffer memory (in bytes)
  // from the system property sun.nio.MaxDirectMemorySize set by the VM.
  // The system property will be removed.
  String s = (String)props.remove("sun.nio.MaxDirectMemorySize");
  if (s != null) {
      if (s.equals("-1")) {
         // -XX:MaxDirectMemorySize not given, take default
          directMemory = Runtime.getRuntime().maxMemory();
      } else {
         long l = Long.parseLong(s);
          if (l > -1)
              directMemory = l;
      }
  }

测试似乎支持这种说法，
“test.java.nio.Buffer.LimitDirectMemory.java”：（

来自 https://github.com/frohoff/jdk8u-dev-jdk/blob/da0da73ab82ed714dc5be94acd2f0d00fbdfe2e9/test/java/nio/Buffer/LimitDirectMemory.java#L74)

 if (size.equals("DEFAULT"))
            return (int)Runtime.getRuntime().maxMemory();