JDK源码阅读-DirectByteBuffer

public static ByteBuffer allocateDirect(int capacity) {
    return new DirectByteBuffer(capacity);
}

DirectByteBuffer(int cap) {                   // package-private
	// 初始化Buffer的四个核心属性
    super(-1, 0, cap, cap);
    // 判断是否需要页面对齐，通过参数-XX:+PageAlignDirectMemory控制，默认为false
    boolean pa = VM.isDirectMemoryPageAligned();
    int ps = Bits.pageSize();
    // 确保有足够内存
    long size = Math.max(1L, (long)cap + (pa ? ps : 0));
    Bits.reserveMemory(size, cap);

    long base = 0;
    try {
        // 调用unsafe方法分配内存
        base = unsafe.allocateMemory(size);
    } catch (OutOfMemoryError x) {
        // 分配失败，释放内存
        Bits.unreserveMemory(size, cap);
        throw x;
    }
    // 初始化内存空间为0
    unsafe.setMemory(base, size, (byte) 0);
    // 设置内存起始地址
    if (pa && (base % ps != 0)) {
        address = base + ps - (base & (ps - 1));
    } else {
        address = base;
    }
    // 使用Cleaner机制注册内存回收处理函数
    cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
    att = null;
}

// 该方法主要用于判断申请的堆外内存是否超过了用例指定的最大值
// 如果还有足够空间可以申请，则更新对应的变量
// 如果已经没有空间可以申请，则抛出OOME
// 参数解释：
//     size：根据是否按页对齐，得到的真实需要申请的内存大小
//     cap：用户指定需要的内存大小(<=size)
static void reserveMemory(long size, int cap) {
    // 因为涉及到更新多个静态统计变量，这里需要Bits类锁
    synchronized (Bits.class) {
        // 获取最大可以申请的对外内存大小，默认值是64MB
        // 可以通过参数-XX:MaxDirectMemorySize=<size>设置这个大小
        if (!memoryLimitSet && VM.isBooted()) {
            maxMemory = VM.maxDirectMemory();
            memoryLimitSet = true;
        }
        // -XX:MaxDirectMemorySize限制的是用户申请的大小，而不考虑对齐情况
        // 所以使用两个变量来统计：
        //     reservedMemory：真实的目前保留的空间
        //     totalCapacity：目前用户申请的空间
        if (cap <= maxMemory - totalCapacity) {
            reservedMemory += size;
            totalCapacity += cap;
            count++;
            return; // 如果空间足够，更新统计变量后直接返回
        }
    }

    // 如果已经没有足够空间，则尝试GC
    System.gc();
    try {
        Thread.sleep(100);
    } catch (InterruptedException x) {
        // Restore interrupt status
        Thread.currentThread().interrupt();
    }
    synchronized (Bits.class) {
        // GC后再次判断，如果还是没有足够空间，则抛出OOME
        if (totalCapacity + cap > maxMemory)
            throw new OutOfMemoryError("Direct buffer memory");
        reservedMemory += size;
        totalCapacity += cap;
        count++;
    }
}

static synchronized void unreserveMemory(long size, int cap) {
    if (reservedMemory > 0) {
        reservedMemory -= size;
        totalCapacity -= cap;
        count--;
        assert (reservedMemory > -1);
    }
}

// 申请一块本地内存。内存空间是未初始化的，其内容是无法预期的。
// 使用freeMemory释放内存，使用reallocateMemory修改内存大小
public native long allocateMemory(long bytes);

// openjdk8/hotspot/src/share/vm/prims/unsafe.cpp
UNSAFE_ENTRY(jlong, Unsafe_AllocateMemory(JNIEnv *env, jobject unsafe, jlong size))
  UnsafeWrapper("Unsafe_AllocateMemory");
  size_t sz = (size_t)size;
  if (sz != (julong)size || size < 0) {
    THROW_0(vmSymbols::java_lang_IllegalArgumentException());
  }
  if (sz == 0) {
    return 0;
  }
  sz = round_to(sz, HeapWordSize);
  // 调用os::malloc申请内存，内部使用malloc函数申请内存
  void* x = os::malloc(sz, mtInternal);
  if (x == NULL) {
    THROW_0(vmSymbols::java_lang_OutOfMemoryError());
  }
  //Copy::fill_to_words((HeapWord*)x, sz / HeapWordSize);
  return addr_to_java(x);
UNSAFE_END

// 使用Cleaner机制注册内存回收处理函数
cleaner = Cleaner.create(this, new Deallocator(base, size, cap));

private static class Deallocator
    implements Runnable
    {

        private static Unsafe unsafe = Unsafe.getUnsafe();

        private long address;
        private long size;
        private int capacity;

        private Deallocator(long address, long size, int capacity) {
            assert (address != 0);
            this.address = address;
            this.size = size;
            this.capacity = capacity;
        }

        public void run() {
            if (address == 0) {
                // Paranoia
                return;
            }
            // 使用unsafe方法释放内存
            unsafe.freeMemory(address);
            address = 0;
            // 更新统计变量
            Bits.unreserveMemory(size, capacity);
        }

    }

public ByteBuffer put(int i, byte x) {
    unsafe.putByte(ix(checkIndex(i)), ((x)));
    return this;
}

public byte get(int i) {
    return ((unsafe.getByte(ix(checkIndex(i)))));
}

private long ix(int i) {
    return address + (i << 0);
}

// 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;
}

public static void saveAndRemoveProperties(Properties props) {
    if (booted)
        throw new IllegalStateException("System initialization has completed");

    savedProps.putAll(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.
    // 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;
        }
    }
    
    //...
}