堆外内存整理

堆外内存, JDK 1.4 nio引进了ByteBuffer.allocateDirect()分配堆外内存

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

• DirectByteBuffer
  DirectByteBuffer(int cap) {// package-private
  super(-1, 0, cap, cap);
  boolean pa = VM.isDirectMemoryPageAligned();//内存是否按页分配对齐
  int ps = Bits.pageSize();//获取每页内存大小
  long size = Math.max(1L, (long)cap + (pa ? ps : 0));//分配内存的大小，如果是按页对齐方式，需要再加一页内存的容量
  重点：分配内存和释放内存之前必须调用此方法
  Bits.reserveMemory(size, cap);//用Bits类保存总分配内存(按页分配)的大小和实际内存的大小
  long base = 0;
  try {//在堆外内存的基地址，指定内存大小
  base = unsafe.allocateMemory(size);//unsafe.cpp中调用os::malloc分配内存
  } catch (OutOfMemoryError x) {
  Bits.unreserveMemory(size, cap);
  throw x;
  }
  unsafe.setMemory(base, size, (byte) 0);
  if (pa && (base % ps != 0)) {//计算堆外内存的基地址
  // Round up to page boundary
  address = base + ps - (base & (ps - 1));
  } else {
  address = base;
  }
  cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
  att = null;
  }

• Deallocator
  private static class Deallocator implements Runnable
  {
  private static Unsafe unsafe = Unsafe.getUnsafe();
  private long address;//基地址
  private long size;//保存了堆外内存的数据（开始地址、大小和容量）
  private int capacity;//保存了堆外内存的数据（开始地址、大小和容量）
  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;
  }
  unsafe.freeMemory(address);//调用OS的方法释放地址，os::free
  address = 0;
  Bits.unreserveMemory(size, capacity);//统计堆外内存大小
  }
  }

• Cleaner
  public class Cleaner extends PhantomReference