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本文主要研究一下openjdk的BufferPoolMXBean

PlatformManagedObject

java.management/java/lang/management/PlatformManagedObject.java

public interface PlatformManagedObject {
    /**
     * Returns an {@link ObjectName ObjectName} instance representing
     * the object name of this platform managed object.
     *
     * @return an {@link ObjectName ObjectName} instance representing
     * the object name of this platform managed object.
     */
    public ObjectName getObjectName();
}

PlatformManagedObject接口定义了getObjectName方法用于返回ObjectName

BufferPoolMXBean

java.management/java/lang/management/BufferPoolMXBean.java

public interface BufferPoolMXBean extends PlatformManagedObject {

    /**
     * Returns the name representing this buffer pool.
     *
     * @return  The name of this buffer pool.
     */
    String getName();

    /**
     * Returns an estimate of the number of buffers in the pool.
     *
     * @return  An estimate of the number of buffers in this pool
     */
    long getCount();

    /**
     * Returns an estimate of the total capacity of the buffers in this pool.
     * A buffer's capacity is the number of elements it contains and the value
     * returned by this method is an estimate of the total capacity of buffers
     * in the pool in bytes.
     *
     * @return  An estimate of the total capacity of the buffers in this pool
     *          in bytes
     */
    long getTotalCapacity();

    /**
     * Returns an estimate of the memory that the Java virtual machine is using
     * for this buffer pool. The value returned by this method may differ
     * from the estimate of the total {@link #getTotalCapacity capacity} of
     * the buffers in this pool. This difference is explained by alignment,
     * memory allocator, and other implementation specific reasons.
     *
     * @return  An estimate of the memory that the Java virtual machine is using
     *          for this buffer pool in bytes, or {@code -1L} if an estimate of
     *          the memory usage is not available
     */
    long getMemoryUsed();
}

• BufferPoolMXBean接口继承了PlatformManagedObject，它定义了getName、getCount、getTotalCapacity、getMemoryUsed方法

ManagementFactoryHelper

java.management/sun/management/ManagementFactoryHelper.java

public class ManagementFactoryHelper {
    static {
        // make sure that the management lib is loaded within
        // java.lang.management.ManagementFactory
        jdk.internal.misc.Unsafe.getUnsafe().ensureClassInitialized(ManagementFactory.class);
    }

    private static final VMManagement jvm = new VMManagementImpl();

    private ManagementFactoryHelper() {};

    public static VMManagement getVMManagement() {
        return jvm;
    }

    static final String LOGGING_MXBEAN_NAME = "java.util.logging:type=Logging";
    private static ClassLoadingImpl    classMBean = null;
    private static MemoryImpl          memoryMBean = null;
    private static ThreadImpl          threadMBean = null;
    private static RuntimeImpl         runtimeMBean = null;
    private static CompilationImpl     compileMBean = null;
    private static BaseOperatingSystemImpl osMBean = null;

    //......

    private static List<BufferPoolMXBean> bufferPools = null;
    public static synchronized List<BufferPoolMXBean> getBufferPoolMXBeans() {
        if (bufferPools == null) {
            bufferPools = new ArrayList<>(2);
            bufferPools.add(createBufferPoolMXBean(SharedSecrets.getJavaNioAccess()
                .getDirectBufferPool()));
            bufferPools.add(createBufferPoolMXBean(sun.nio.ch.FileChannelImpl
                .getMappedBufferPool()));
        }
        return bufferPools;
    }

        private static BufferPoolMXBean
        createBufferPoolMXBean(final JavaNioAccess.BufferPool pool)
    {
        return new BufferPoolMXBean() {
            private volatile ObjectName objname;  // created lazily
            @Override
            public ObjectName getObjectName() {
                ObjectName result = objname;
                if (result == null) {
                    synchronized (this) {
                        result = objname;
                        if (result == null) {
                            result = Util.newObjectName(BUFFER_POOL_MXBEAN_NAME +
                                ",name=" + pool.getName());
                            objname = result;
                        }
                    }
                }
                return result;
            }
            @Override
            public String getName() {
                return pool.getName();
            }
            @Override
            public long getCount() {
                return pool.getCount();
            }
            @Override
            public long getTotalCapacity() {
                return pool.getTotalCapacity();
            }
            @Override
            public long getMemoryUsed() {
                return pool.getMemoryUsed();
            }
        };
    }

    //......
}

• ManagementFactoryHelper的getBufferPoolMXBeans方法会通过createBufferPoolMXBean方法创建两个BufferPoolMXBean，然后添加到bufferPools
• 其中一个是DirectBufferPool，一个是MappedBufferPool；他们分别使用SharedSecrets.getJavaNioAccess().getDirectBufferPool()以及sun.nio.ch.FileChannelImpl.getMappedBufferPool()来创建
• createBufferPoolMXBean方法使用匿名类创建了BufferPoolMXBean的实现；createBufferPoolMXBean方法接收JavaNioAccess.BufferPool参数，其getCount、getTotalCapacity、getMemoryUsed等均是直接使用pool的相关方法

JavaNioAccess.BufferPool

java.base/jdk/internal/access/JavaNioAccess.java

public interface JavaNioAccess {
    /**
     * Provides access to information on buffer usage.
     */
    interface BufferPool {
        String getName();
        long getCount();
        long getTotalCapacity();
        long getMemoryUsed();
    }
    BufferPool getDirectBufferPool();
}

• JavaNioAccess里头定义了BufferPool接口，它定义了getName、getCount、getTotalCapacity、getMemoryUsed方法；除此之外JavaNioAccess还定义了getDirectBufferPool方法用于返回BufferPool

SharedSecrets

java.base/jdk/internal/access/SharedSecrets.java

public class SharedSecrets {
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static JavaUtilJarAccess javaUtilJarAccess;
    private static JavaLangAccess javaLangAccess;
    private static JavaLangModuleAccess javaLangModuleAccess;
    private static JavaLangInvokeAccess javaLangInvokeAccess;
    private static JavaLangRefAccess javaLangRefAccess;
    private static JavaIOAccess javaIOAccess;
    private static JavaNetInetAddressAccess javaNetInetAddressAccess;
    private static JavaNetHttpCookieAccess javaNetHttpCookieAccess;
    private static JavaNetSocketAccess javaNetSocketAccess;
    private static JavaNetUriAccess javaNetUriAccess;
    private static JavaNetURLAccess javaNetURLAccess;
    private static JavaNetURLClassLoaderAccess javaNetURLClassLoaderAccess;
    private static JavaNioAccess javaNioAccess;
    private static JavaIOFileDescriptorAccess javaIOFileDescriptorAccess;
    private static JavaIOFilePermissionAccess javaIOFilePermissionAccess;
    private static JavaSecurityAccess javaSecurityAccess;
    private static JavaUtilZipFileAccess javaUtilZipFileAccess;
    private static JavaUtilResourceBundleAccess javaUtilResourceBundleAccess;
    private static JavaAWTAccess javaAWTAccess;
    private static JavaAWTFontAccess javaAWTFontAccess;
    private static JavaBeansAccess javaBeansAccess;
    private static JavaObjectInputStreamAccess javaObjectInputStreamAccess;
    private static JavaObjectInputFilterAccess javaObjectInputFilterAccess;
    private static JavaIORandomAccessFileAccess javaIORandomAccessFileAccess;
    private static JavaxCryptoSealedObjectAccess javaxCryptoSealedObjectAccess;

    //......

    public static void setJavaNioAccess(JavaNioAccess jna) {
        javaNioAccess = jna;
    }

    public static JavaNioAccess getJavaNioAccess() {
        if (javaNioAccess == null) {
            // Ensure java.nio.Buffer is initialized, which provides the
            // shared secret.
            unsafe.ensureClassInitialized(java.nio.Buffer.class);
        }
        return javaNioAccess;
    }

    //......
}

• SharedSecrets提供了JavaNioAccess的getter及setter

Buffer

java.base/java/nio/Buffer.java

public abstract class Buffer {
    // Cached unsafe-access object
    static final Unsafe UNSAFE = Unsafe.getUnsafe();

    /**
     * The characteristics of Spliterators that traverse and split elements
     * maintained in Buffers.
     */
    static final int SPLITERATOR_CHARACTERISTICS =
        Spliterator.SIZED | Spliterator.SUBSIZED | Spliterator.ORDERED;

    // Invariants: mark <= position <= limit <= capacity
    private int mark = -1;
    private int position = 0;
    private int limit;
    private int capacity;

    // Used by heap byte buffers or direct buffers with Unsafe access
    // For heap byte buffers this field will be the address relative to the
    // array base address and offset into that array. The address might
    // not align on a word boundary for slices, nor align at a long word
    // (8 byte) boundary for byte[] allocations on 32-bit systems.
    // For direct buffers it is the start address of the memory region. The
    // address might not align on a word boundary for slices, nor when created
    // using JNI, see NewDirectByteBuffer(void*, long).
    // Should ideally be declared final
    // NOTE: hoisted here for speed in JNI GetDirectBufferAddress
    long address;

    //......

    static {
        // setup access to this package in SharedSecrets
        SharedSecrets.setJavaNioAccess(
            new JavaNioAccess() {
                @Override
                public JavaNioAccess.BufferPool getDirectBufferPool() {
                    return Bits.BUFFER_POOL;
                }
            });
    }

}

• 抽象类Buffer有个static代码块，里头创建了匿名JavaNioAccess，然后设置到了SharedSecrets中；其中匿名JavaNioAccess的getDirectBufferPool方法返回的是Bits.BUFFER_POOL

FileChannelImpl

java.base/sun/nio/ch/FileChannelImpl.java

public class FileChannelImpl
    extends FileChannel
{
    // Memory allocation size for mapping buffers
    private static final long allocationGranularity;

    // Access to FileDescriptor internals
    private static final JavaIOFileDescriptorAccess fdAccess =
        SharedSecrets.getJavaIOFileDescriptorAccess();

    // Used to make native read and write calls
    private final FileDispatcher nd;

    // File descriptor
    private final FileDescriptor fd;

    //......

    // -- Memory-mapped buffers --

    private static class Unmapper
        implements Runnable
    {
        // may be required to close file
        private static final NativeDispatcher nd = new FileDispatcherImpl();

        // keep track of mapped buffer usage
        static volatile int count;
        static volatile long totalSize;
        static volatile long totalCapacity;

        private volatile long address;
        private final long size;
        private final int cap;
        private final FileDescriptor fd;

        private Unmapper(long address, long size, int cap,
                         FileDescriptor fd)
        {
            assert (address != 0);
            this.address = address;
            this.size = size;
            this.cap = cap;
            this.fd = fd;

            synchronized (Unmapper.class) {
                count++;
                totalSize += size;
                totalCapacity += cap;
            }
        }

        public void run() {
            if (address == 0)
                return;
            unmap0(address, size);
            address = 0;

            // if this mapping has a valid file descriptor then we close it
            if (fd.valid()) {
                try {
                    nd.close(fd);
                } catch (IOException ignore) {
                    // nothing we can do
                }
            }

            synchronized (Unmapper.class) {
                count--;
                totalSize -= size;
                totalCapacity -= cap;
            }
        }
    }

    //......

    /**
     * Invoked by sun.management.ManagementFactoryHelper to create the management
     * interface for mapped buffers.
     */
    public static JavaNioAccess.BufferPool getMappedBufferPool() {
        return new JavaNioAccess.BufferPool() {
            @Override
            public String getName() {
                return "mapped";
            }
            @Override
            public long getCount() {
                return Unmapper.count;
            }
            @Override
            public long getTotalCapacity() {
                return Unmapper.totalCapacity;
            }
            @Override
            public long getMemoryUsed() {
                return Unmapper.totalSize;
            }
        };
    }

    //......
}

FileChannelImpl定义了getMappedBufferPool方法，返回的是匿名JavaNioAccess.BufferPool，其相关返回实现直接使用Unmapper的对应方法；Unmapper实现了Runnable接口

小结

• BufferPoolMXBean接口继承了PlatformManagedObject，它定义了getName、getCount、getTotalCapacity、getMemoryUsed方法
• ManagementFactoryHelper的getBufferPoolMXBeans方法会通过createBufferPoolMXBean方法创建两个BufferPoolMXBean，然后添加到bufferPools；其中一个是DirectBufferPool，一个是MappedBufferPool；他们分别使用SharedSecrets.getJavaNioAccess().getDirectBufferPool()以及sun.nio.ch.FileChannelImpl.getMappedBufferPool()来创建
• createBufferPoolMXBean方法接收JavaNioAccess.BufferPool参数；抽象类Buffer有个static代码块，里头创建了匿名JavaNioAccess，然后设置到了SharedSecrets中；其中匿名JavaNioAccess的getDirectBufferPool方法返回的是Bits.BUFFER_POOL；FileChannelImpl定义了getMappedBufferPool方法，返回的是匿名JavaNioAccess.BufferPool，其相关返回实现直接使用Unmapper的对应方法；Unmapper实现了Runnable接口

doc

• Interface BufferPoolMXBean