虚引用与堆外内存研究(基于DirectByteBuffer源码)

阳光穿透心脏的1/2处 2023-01-10 10:23 149阅读 0赞

(1)堆内存  
ByteBuffer buffer = ByteBuffer.allocate(1024);  
字节数组保存数据HeapByteBuffer  
  
(2)堆外内存  
ByteBuffer directByteBuffer = ByteBuffer.allocateDirect(1024);  
Buffer类有个：long address;用于保存堆外空间地址  
unsafe来分配以及最后回收空间  
构建Cleaner对象，继承虚引用，将当前堆外内存以及垃圾清理线程对象传递过去，GC发生后，调用Deallocator的clean方法，内部有调用unsafe来回收堆外内存。

DirectByteBuffer(int cap) \{  
//\*\* address，base分配堆外内存空间之后的起始地址  
long base = 0;  
try \{  
base = unsafe.allocateMemory(size);  
\} catch (OutOfMemoryError x) \{  
Bits.unreserveMemory(size, cap);  
throw x;  
\}  
unsafe.setMemory(base, size, (byte) 0);  
if (pa && (base % ps != 0)) \{  
// Round up to page boundary  
address = base + ps - (base & (ps - 1));  
\} else \{  
address = base;  
\}  
//构建Cleaner对象用于跟踪DirectByteBuffer对象的垃圾回收，以实现当DirectByteBuffer被垃圾回收时，堆外内存也会被释放  
cleaner = Cleaner.create(this, new Deallocator(base, size, cap));  
//\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*  
\}  
/\*\*  
\*\*继承虚引用  
\* clener回收列表：其中first是Cleaner类的静态变量，Cleaner对象在初始化时会被添加到Clener链表中， \* 和first形成引用关系，。  
\* ReferenceQueue是用来保存需要回收的Cleaner对象。  
\* cleaner对象唯一保存了堆外内存的数据（开始地址、大小和容量）  
\* 若DerectByteBuffer对象被回收，那么把它内部的Cleaner对象放入到ReferenceQueue中，并触发clean \* 方法。释放堆外内存。  
\*/  
public class Cleaner extends PhantomReference<Object> \{  
private static final ReferenceQueue<Object> dummyQueue = new ReferenceQueue();  
private static Cleaner first = null;  
private Cleaner next = null;  
private Cleaner prev = null;  
private final Runnable thunk;  
  
// var1 即directBuffer, var2即runnable对象Deallocator对象  
private Cleaner(Object var1, Runnable var2) \{  
super(var1, dummyQueue);  
this.thunk = var2;  
\}  
public static Cleaner create(Object var0, Runnable var1) \{  
return var1 == null ? null : add(new Cleaner(var0, var1));  
\}  
public void clean() \{  
//清理  
if (remove(this)) \{  
try \{  
this.thunk.run();  
\} catch (final Throwable var2) \{  
\*\*\*  
\}  
\}  
\}  
\}  
// 回收线程，保存堆外内存信息以及执行回收操作  
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) \{  
return;  
\}  
unsafe.freeMemory(address);  
address = 0;  
Bits.unreserveMemory(size, capacity);  
\}  
\}