聊聊jvm的-XX:MaxDirectMemorySize

最新推荐文章于 2025-09-06 15:27:13 发布

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最新推荐文章于 2025-09-06 15:27:13 发布

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文章标签： java netty 大数据

原文链接：https://juejin.im/post/5c9ced366fb9a070e344c614

本文详细探讨了JVM的-XX:MaxDirectMemorySize参数，介绍了其作用和默认值，以及如何通过System.initPhase1、VM.saveProperties等方法获取最大直接内存大小。同时，通过实例展示了如何使用API监控directBuffer，以及在出现OOM时的处理。文章还提到了Java 9以后模块化对VM和SharedSecrets的影响，并利用NMT查看directBuffer使用情况。

序

本文主要研究一下jvm的-XX:MaxDirectMemorySize

-XX:MaxDirectMemorySize

-XX:MaxDirectMemorySize=size用于设置New I/O(java.nio) direct-buffer allocations的最大大小，size的单位可以使用k/K、m/M、g/G；如果没有设置该参数则默认值为0，意味着JVM自己自动给NIO direct-buffer allocations选择最大大小

System.initPhase1

java.base/java/lang/System.java

public final class System {
    /* Register the natives via the static initializer.
     *
     * VM will invoke the initializeSystemClass method to complete
     * the initialization for this class separated from clinit.
     * Note that to use properties set by the VM, see the constraints
     * described in the initializeSystemClass method.
     */
    private static native void registerNatives();
    static {
        registerNatives();
    }

    /** Don't let anyone instantiate this class */
    private System() {
    }

    /**
     * Initialize the system class.  Called after thread initialization.
     */
    private static void initPhase1() {
        // VM might invoke JNU_NewStringPlatform() to set those encoding
        // sensitive properties (user.home, user.name, boot.class.path, etc.)
        // during "props" initialization.
        // The charset is initialized in System.c and does not depend on the Properties.
        Map<String, String> tempProps = SystemProps.initProperties();
        VersionProps.init(tempProps);

        // There are certain system configurations that may be controlled by
        // VM options such as the maximum amount of direct memory and
        // Integer cache size used to support the object identity semantics
        // of autoboxing.  Typically, the library will obtain these values
        // from the properties set by the VM.  If the properties are for
        // internal implementation use only, these properties should be
        // masked from the system properties.
        //
        // Save a private copy of the system properties object that
        // can only be accessed by the internal implementation.
        VM.saveProperties(tempProps);
        props = createProperties(tempProps);

        StaticProperty.javaHome();          // Load StaticProperty to cache the property values

        lineSeparator = props.getProperty("line.separator");

        FileInputStream fdIn = new FileInputStream(FileDescriptor.in);
        FileOutputStream fdOut = new FileOutputStream(FileDescriptor.out);
        FileOutputStream fdErr = new FileOutputStream(FileDescriptor.err);
        setIn0(new BufferedInputStream(fdIn));
        setOut0(newPrintStream(fdOut, props.getProperty("sun.stdout.encoding")));
        setErr0(newPrintStream(fdErr, props.getProperty("sun.stderr.encoding")));

        // Setup Java signal handlers for HUP, TERM, and INT (where available).
        Terminator.setup();

        // Initialize any miscellaneous operating system settings that need to be
        // set for the class libraries. Currently this is no-op everywhere except
        // for Windows where the process-wide error mode is set before the java.io
        // classes are used.
        VM.initializeOSEnvironment();

        // The main thread is not added to its thread group in the same
        // way as other threads; we must do it ourselves here.
        Thread current = Thread.currentThread();
        current.getThreadGroup().add(current);

        // register shared secrets
        setJavaLangAccess();

        // Subsystems that are invoked during initialization can invoke
        // VM.isBooted() in order to avoid doing things that should
        // wait until the VM is fully initialized. The initialization level
        // is incremented from 0 to 1 here to indicate the first phase of
        // initialization has completed.
        // IMPORTANT: Ensure that this remains the last initialization action!
        VM.initLevel(1);
    }

    //......
}
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System的initPhase1方法会调用VM.saveProperties(tempProps)方法来保存一份系统配置供内部实现使用；其中tempProps为SystemProps.initProperties()

jvm.cpp

hotspot/share/prims/jvm.cpp

  // Convert the -XX:MaxDirectMemorySize= command line flag
  // to the sun.nio.MaxDirectMemorySize property.
  // Do this after setting user properties to prevent people
  // from setting the value with a -D option, as requested.
  // Leave empty if not supplied
  if (!FLAG_IS_DEFAULT(MaxDirectMemorySize)) {
    char as_chars[256];
    jio_snprintf(as_chars, sizeof(as_chars), JULONG_FORMAT, MaxDirectMemorySize);
    Handle key_str = java_lang_String::create_from_platform_dependent_str("sun.nio.MaxDirectMemorySize", CHECK_NULL);
    Handle value_str  = java_lang_String::create_from_platform_dependent_str(as_chars, CHECK_NULL);
    result_h->obj_at_put(ndx * 2,  key_str());
    result_h->obj_at_put(ndx * 2 + 1, value_str());
    ndx++;
  }
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jvm.cpp里头有一段代码用于把-XX:MaxDirectMemorySize命令参数转换为key为sun.nio.MaxDirectMemorySize的属性

VM.saveProperties

java.base/jdk/internal/misc/VM.java

public class VM {

    // the init level when the VM is fully initialized
    private static final int JAVA_LANG_SYSTEM_INITED     = 1;
    private static final int MODULE_SYSTEM_INITED        = 2;
    private static final int SYSTEM_LOADER_INITIALIZING  = 3;
    private static final int SYSTEM_BOOTED               = 4;
    private static final int SYSTEM_SHUTDOWN             = 5;


    // 0, 1, 2, ...
    private static volatile int initLevel;
    private static final Object lock = new Object();

    //......

    // 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;

    // Returns the maximum amount of allocatable direct buffer memory.
    // The directMemory variable is initialized during system initialization
    // in the saveAndRemoveProperties method.
    //
    public static long maxDirectMemory() {
        return directMemory;
    }

    //......

    // Save a private copy of the system properties and remove
    // the system properties that are not intended for public access.
    //
    // This method can only be invoked during system initialization.
    public static void saveProperties(Map<String, String> props) {
        if (initLevel() != 0)
            throw new IllegalStateException("Wrong init level");

        // only main thread is running at this time, so savedProps and
        // its content will be correctly published to threads started later
        if (savedProps == null) {
            savedProps = props;
        }

        // 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.
        // If not set or set to -1, the max memory will be used
        // The system property will be removed.
        String s = props.get("sun.nio.MaxDirectMemorySize");
        if (s == null || s.isEmpty() || s.equals("-1")) {
            // -XX:MaxDirectMemorySize not given, take default
            directMemory = Runtime.getRuntime().maxMemory();
        } else {
            long l = Long.parseLong(s);
            if (l > -1)
                directMemory = l;
        }

        // Check if direct buffers should be page aligned
        s = props.get("sun.nio.PageAlignDirectMemory");
        if ("true".equals(s))
            pageAlignDirectMemory = true;
    }

    //......
}
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VM的saveProperties方法读取sun.nio.MaxDirectMemorySize属性，如果为null或者是空或者是-1，那么则设置为Runtime.getRuntime().maxMemory()；如果有设置MaxDirectMemorySize且值大于-1，那么使用该值作为directMemory的值；而VM的maxDirectMemory方法则返回的是directMemory的值

获取maxDirectMemory的值

实例

    public BufferPoolMXBean getDirectBufferPoolMBean(){
        return ManagementFactory.getPlatformMXBeans(BufferPoolMXBean.class)
                .stream()
                .filter(e -> e.getName().equals("direct"))
                .findFirst()
                .orElseThrow();
    }

    public JavaNioAccess.BufferPool getNioBufferPool(){
        return SharedSecrets.getJavaNioAccess().getDirectBufferPool();
    }

    /**
     * -XX:MaxDirectMemorySize=60M
     */
    @Test
    public void testGetMaxDirectMemory(){
        ByteBuffer.allocateDirect(25*1024*1024);
        System.out.println(Runtime.getRuntime().maxMemory() / 1024.0 / 1024.0);
        System.out.println(VM.maxDirectMemory() / 1024.0 / 1024.0);
        System.out.println(getDirectBufferPoolMBean().getTotalCapacity() / 1024.0 / 1024.0);
        System.out.println(getNioBufferPool().getTotalCapacity() / 1024.0 / 1024.0);
    }
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输出

输出结果如下：

4096.0
60.0
25.0
25.0
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由于java9模块化之后，VM从原来的sun.misc.VM变更到java.base模块下的jdk.internal.misc.VM；上面代码默认是unamed module，要使用jdk.internal.misc.VM就需要使用--add-exports java.base/jdk.internal.misc=ALL-UNNAMED将其导出到UNNAMED，这样才可以运行
同理java9模块化之后，SharedSecrets从原来的sun.misc.SharedSecrets变更到java.base模块下的jdk.internal.access.SharedSecrets；要使用--add-exports java.base/jdk.internal.access=ALL-UNNAMED将其导出到UNNAMED，这样才可以运行
从输出结果可以看出，Runtime.getRuntime().maxMemory()输出的值是正确的，而BufferPoolMXBean及JavaNioAccess.BufferPool的getTotalCapacity返回的都是directBuffer大小，而非max值

使用API查看directBuffer使用情况

实例

    /**
     * -XX:MaxDirectMemorySize=60M
     */
    @Test
    public void testGetDirectMemoryUsage(){
        ByteBuffer.allocateDirect(30*1024*1024);
        System.out.println(getDirectBufferPoolMBean().getMemoryUsed() / 1024.0 / 1024.0);
        System.out.println(getNioBufferPool().getMemoryUsed() / 1024.0 / 1024.0);
    }
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输出

输出结果如下：

30.0
30.0
复制代码

可以看到BufferPoolMXBean及JavaNioAccess.BufferPool的getMemoryUsed可以返回directBuffer大小

OOM

java.lang.OutOfMemoryError: Direct buffer memory

	at java.base/java.nio.Bits.reserveMemory(Bits.java:175)
	at java.base/java.nio.DirectByteBuffer.<init>(DirectByteBuffer.java:118)
	at java.base/java.nio.ByteBuffer.allocateDirect(ByteBuffer.java:317)
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如果上面的ByteBuffer.allocateDirect改为分配超过60M，则运行抛出OutOfMemoryError

使用NMT查看directBuffer使用情况

jcmd 3088 VM.native_memory scale=MB
3088:

Native Memory Tracking:

Total: reserved=5641MB, committed=399MB
-                 Java Heap (reserved=4096MB, committed=258MB)
                            (mmap: reserved=4096MB, committed=258MB)

-                     Class (reserved=1032MB, committed=6MB)
                            (classes #1609)
                            (  instance classes #1460, array classes #149)
                            (mmap: reserved=1032MB, committed=5MB)
                            (  Metadata:   )
                            (    reserved=8MB, committed=5MB)
                            (    used=3MB)
                            (    free=2MB)
                            (    waste=0MB =0.00%)
                            (  Class space:)
                            (    reserved=1024MB, committed=1MB)
                            (    used=0MB)
                            (    free=0MB)
                            (    waste=0MB =0.00%)

-                    Thread (reserved=18MB, committed=18MB)
                            (thread #18)
                            (stack: reserved=18MB, committed=18MB)

-                      Code (reserved=242MB, committed=7MB)
                            (mmap: reserved=242MB, committed=7MB)

-                        GC (reserved=203MB, committed=60MB)
                            (malloc=18MB #2443)
                            (mmap: reserved=185MB, committed=43MB)

-                  Internal (reserved=1MB, committed=1MB)
                            (malloc=1MB #1257)

-                     Other (reserved=30MB, committed=30MB)
                            (malloc=30MB #2)

-                    Symbol (reserved=1MB, committed=1MB)
                            (malloc=1MB #13745)

-        Shared class space (reserved=17MB, committed=17MB)
                            (mmap: reserved=17MB, committed=17MB)
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从Other部分可以看到其值跟ByteBuffer.allocateDirect使用的值一致，改变ByteBuffer.allocateDirect的值再重新查看，可以发现Other部分跟着改变；因而初步断定Other部分应该是可以反映direct memory的使用大小

小结

-XX:MaxDirectMemorySize=size用于设置New I/O(java.nio) direct-buffer allocations的最大大小，size的单位可以使用k/K、m/M、g/G；如果没有设置该参数则默认值为0，意味着JVM自己自动给NIO direct-buffer allocations选择最大大小；从代码java.base/jdk/internal/misc/VM.java中可以看到默认是取的Runtime.getRuntime().maxMemory()
使用jdk.internal.misc.VM.maxDirectMemory()可以获取maxDirectMemory的值；由于java9模块化之后，VM从原来的sun.misc.VM变更到java.base模块下的jdk.internal.misc.VM；上面代码默认是unamed module，要使用jdk.internal.misc.VM就需要使用--add-exports java.base/jdk.internal.misc=ALL-UNNAMED将其导出到UNNAMED，这样才可以运行
BufferPoolMXBean及JavaNioAccess.BufferPool(通过SharedSecrets获取)的getMemoryUsed可以获取direct memory的大小；其中java9模块化之后，SharedSecrets从原来的sun.misc.SharedSecrets变更到java.base模块下的jdk.internal.access.SharedSecrets；要使用--add-exports java.base/jdk.internal.access=ALL-UNNAMED将其导出到UNNAMED，这样才可以运行