裝飾器：裝飾器模式主要用于系統(tǒng)擴張功能用，在系統(tǒng)原有的功能上，擴展出其他的功能，JDK中IO包用到很多，比如datainputstream之類，需要用其他流進行構(gòu)造的上層類，符合面向?qū)ο笤O計的開閉原則
下面我來寫個例子：
首先，寫一個Car模版，定義基本屬性及行為功能driver
package com.google.desginpattern.decoration;
//其實這是個模版
public abstract class Car {
private int spreed;
public int getSpreed() {
return spreed;
}
public void setSpreed(int spreed) {
this.spreed = spreed;
}
public abstract void driver();
}
第二步：具體車比如寶馬，這是目前系統(tǒng)中這個類能提供的功能
package com.google.desginpattern.decoration;
//目前系統(tǒng)中此類的功能
public class BMWCar extends Car {
@Override
public void driver() {
System.out.println("我開著寶馬車");
}
}
現(xiàn)在我想在這個類上擴展出其他功能，比如：泡妞
第三步：定義一個裝飾模板，為什么給他定義個模板呢~因為可以給這個BMWCar類裝飾很不同的功能，不只泡妞一個~
繼承Car父類，覆蓋driver功能，調(diào)用Car引用完成driver功能
package com.google.desginpattern.decoration;
//裝飾器父類
public abstract class DecorationCar extends Car {
// 引入car
private Car car;
@Override
public void driver() {
car.driver();// 調(diào)用此car來完成裝飾器的功能
}
public Car getCar() {
return car;
}
public void setCar(Car car) {
this.car = car;
}
}
第四步：具體的裝飾功能，添加一個構(gòu)造函數(shù)，參數(shù)為Car,為裝飾父類Car引用賦值，其實就是原來具體的功能類，回想下IO包里經(jīng)常new的代碼段就明白~~
package com.google.desginpattern.decoration;
//具體的裝飾類,添加額外的泡妞功能
public class DecorationBMWCar extends DecorationCar {
public DecorationBMWCar(Car car) {
super.setCar(car);
}
@Override
public void driver() {
// TODO Auto-generated method stub
super.driver();// 調(diào)用原來的功能
System.out.println("泡妞");// 添加額外的功能
}
}
測試類：構(gòu)造的方法很像IO包里的流
package com.google.desginpattern.decoration;
public class Test {
public static void main(String[] args) {
Car car = new DecorationBMWCar(new BMWCar());
car.driver();
}
}
輸出：
我開著寶馬車
泡妞
此類采用模板模式設計，此類為一個抽象類，但是沒抽象方法，每個sync子類需要實現(xiàn)5個受保護的方法
這個5個方法在AbstractQueuedSynchronizer 都拋出throw new UnsupportedOperationException();
AbstractQueuedSynchronizer 中有3個屬性：主要聲明一個狀態(tài)和一個wait queue,通過
wait queue中Node 為一個雙向鏈表，需要去理解Node中幾個靜態(tài)字段值的意義，下面為他的源碼：
static final class Node {
/** waitStatus value to indicate thread has cancelled */
static final int CANCELLED = 1;
/** waitStatus value to indicate successor's thread needs unparking */
static final int SIGNAL = -1;
/** waitStatus value to indicate thread is waiting on condition */
static final int CONDITION = -2;
/** Marker to indicate a node is waiting in shared mode */
static final Node SHARED = new Node();
/** Marker to indicate a node is waiting in exclusive mode */
static final Node EXCLUSIVE = null;
/**
* Status field, taking on only the values:
* SIGNAL: The successor of this node is (or will soon be)
* blocked (via park), so the current node must
* unpark its successor when it releases or
* cancels. To avoid races, acquire methods must
* first indicate they need a signal,
* then retry the atomic acquire, and then,
* on failure, block.
* CANCELLED: This node is cancelled due to timeout or interrupt.
* Nodes never leave this state. In particular,
* a thread with cancelled node never again blocks.
* CONDITION: This node is currently on a condition queue.
* It will not be used as a sync queue node until
* transferred. (Use of this value here
* has nothing to do with the other uses
* of the field, but simplifies mechanics.)
* 0: None of the above
*
* The values are arranged numerically to simplify use.
* Non-negative values mean that a node doesn't need to
* signal. So, most code doesn't need to check for particular
* values, just for sign.
*
* The field is initialized to 0 for normal sync nodes, and
* CONDITION for condition nodes. It is modified only using
* CAS.
*/
volatile int waitStatus;
/**
* Link to predecessor node that current node/thread relies on
* for checking waitStatus. Assigned during enqueing, and nulled
* out (for sake of GC) only upon dequeuing. Also, upon
* cancellation of a predecessor, we short-circuit while
* finding a non-cancelled one, which will always exist
* because the head node is never cancelled: A node becomes
* head only as a result of successful acquire. A
* cancelled thread never succeeds in acquiring, and a thread only
* cancels itself, not any other node.
*/
volatile Node prev;
/**
* Link to the successor node that the current node/thread
* unparks upon release. Assigned once during enqueuing, and
* nulled out (for sake of GC) when no longer needed. Upon
* cancellation, we cannot adjust this field, but can notice
* status and bypass the node if cancelled. The enq operation
* does not assign next field of a predecessor until after
* attachment, so seeing a null next field does not
* necessarily mean that node is at end of queue. However, if
* a next field appears to be null, we can scan prev's from
* the tail to double-check.
*/
volatile Node next;
/**
* The thread that enqueued this node. Initialized on
* construction and nulled out after use.
*/
volatile Thread thread;
/**
* Link to next node waiting on condition, or the special
* value SHARED. Because condition queues are accessed only
* when holding in exclusive mode, we just need a simple
* linked queue to hold nodes while they are waiting on
* conditions. They are then transferred to the queue to
* re-acquire. And because conditions can only be exclusive,
* we save a field by using special value to indicate shared
* mode.
*/
Node nextWaiter;
/**
* Returns true if node is waiting in shared mode
*/
final boolean isShared() {
return nextWaiter == SHARED;
}
/**
* Returns previous node, or throws NullPointerException if
* null. Use when predecessor cannot be null.
* @return the predecessor of this node
*/
final Node predecessor() throws NullPointerException {
Node p = prev;
if (p == null)
throw new NullPointerException();
else
return p;
}
Node() { // Used to establish initial head or SHARED marker
}
Node(Thread thread, Node mode) { // Used by addWaiter
this.nextWaiter = mode;
this.thread = thread;
}
Node(Thread thread, int waitStatus) { // Used by Condition
this.waitStatus = waitStatus;
this.thread = thread;
}
}
獲取鎖定調(diào)用的方法，其實這個方法是阻塞的：
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
如果獲取不成功則調(diào)用如下方法：
final boolean acquireQueued(final Node node, int arg) {
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {//當節(jié)點是頭節(jié)點且獨占時才返回
setHead(node);
p.next = null; // help GC
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())//阻塞并判斷是否打斷，其實這個判斷才是自旋鎖真正的猥瑣點，
意思是如果你的前繼節(jié)點不是head,而且當你的前繼節(jié)點狀態(tài)是Node.SIGNAL時，你這個線程將被park()，直到另外的線程release時,發(fā)現(xiàn)head.next是你這個node時，才unpark，你才能繼續(xù)循環(huán)并獲取鎖
interrupted = true;
}
shouldParkAfterFailedAcquire這個方法刪除所有waitStatus>0也就是CANCELLED狀態(tài)的Node,并設置前繼節(jié)點為signal
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int s = pred.waitStatus;
if (s < 0)
/*
* This node has already set status asking a release
* to signal it, so it can safely park
*/
return true;
if (s > 0) {
/*
* Predecessor was cancelled. Skip over predecessors and
* indicate retry.
*/
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
pred.next = node;
}
else
/*
* Indicate that we need a signal, but don't park yet. Caller
* will need to retry to make sure it cannot acquire before
* parking.
*/
compareAndSetWaitStatus(pred, 0, Node.SIGNAL);
return false;
}
使用LockSupport.park(this)，禁用當前線程
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);//block
return Thread.interrupted();
}
釋放鎖：
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);//unblock
return true;
}
return false;
}
private void unparkSuccessor(Node node) {
/*
* Try to clear status in anticipation of signalling. It is
* OK if this fails or if status is changed by waiting thread.
*/
compareAndSetWaitStatus(node, Node.SIGNAL, 0);
/*
* Thread to unpark is held in successor, which is normally
* just the next node. But if cancelled or apparently null,
* traverse backwards from tail to find the actual
* non-cancelled successor.
*/
Node s = node.next;
if (s == null || s.waitStatus > 0) {
s = null;
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0)
s = t;
}
if (s != null)
LockSupport.unpark(s.thread);
}
} catch (RuntimeException ex) {
cancelAcquire(node);
throw ex;
}
}
看下ReentrantLock鎖中sync的實現(xiàn)：
static abstract class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = -5179523762034025860L;
/**
* Performs {@link Lock#lock}. The main reason for subclassing
* is to allow fast path for nonfair version.
*/
abstract void lock();
/**
* Performs non-fair tryLock. tryAcquire is
* implemented in subclasses, but both need nonfair
* try for trylock method.
*/
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
protected final boolean tryRelease(int releases) {
int c = getState() - releases;
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
if (c == 0) {
free = true;
setExclusiveOwnerThread(null);
}
setState(c);
return free;
}
protected final boolean isHeldExclusively() {
// While we must in general read state before owner,
// we don't need to do so to check if current thread is owner
return getExclusiveOwnerThread() == Thread.currentThread();
}
final ConditionObject newCondition() {
return new ConditionObject();
}
// Methods relayed from outer class
final Thread getOwner() {
return getState() == 0 ? null : getExclusiveOwnerThread();
}
final int getHoldCount() {
return isHeldExclusively() ? getState() : 0;
}
final boolean isLocked() {
return getState() != 0;
}
/**
* Reconstitutes this lock instance from a stream.
* @param s the stream
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
setState(0); // reset to unlocked state
}
}
非公平規(guī)則下nonfairTryAcquire，獲取當前鎖的state,通過CAS原子操作設置為1，并將當前線程設置為獨占線程，如果當前線程已經(jīng)拿了鎖，則state增加1
公平鎖中 有如下判斷：
if (isFirst(current) &&//判斷頭元素
compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
在獲取鎖步驟：
1.調(diào)用tryAcquire來獲取，如果失敗，則進入2
2.調(diào)用addWaiter，以獨占模式將node放到tail位置
3.調(diào)用acquireQueued方法，此方法阻塞，直到node的pre為head,并成功獲取鎖定，也可能存在阻塞并打斷情況
釋放鎖的步驟：
1.放棄排他鎖持有權(quán)
2.unpark 節(jié)點的下一個blocked節(jié)點
公平鎖與非公平鎖：從代碼上看，非公平鎖是讓當前線程優(yōu)先獨占，而公平鎖則是讓等待時間最長的線程優(yōu)先，非公平的可能讓其他線程沒機會執(zhí)行，而公平的則可以讓等待時間最長的先執(zhí)行，但是性能上會差點
linkedhashmap繼承自hashmap,他的底層維護的一個鏈表， private transient Entry<K,V> header 來記錄元素的插入順序和訪問順序;
hashmap的構(gòu)造函數(shù)中調(diào)用init()方法，而linkedhashmap中重寫了init(),將head元素初始化
void init() {
header = new Entry<K,V>(-1, null, null, null);
header.before = header.after = header;
}
private final boolean accessOrder這個屬性表示是否要根據(jù)訪問順序改變線性結(jié)構(gòu)
在linkedhashmap中改寫了hashmap的get()方法，增加了 e.recordAccess(this)，這個方法主要是根據(jù)accessOrder的值判斷是否需要實現(xiàn)LRU，
void recordAccess(HashMap<K,V> m) {
LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m;
if (lm.accessOrder) {
lm.modCount++;
remove();
addBefore(lm.header);
}
}
addBefore這個方法是把剛訪問的元素放到head的前面
private void addBefore(Entry<K,V> existingEntry) {
after = existingEntry;
before = existingEntry.before;
before.after = this;
after.before = this;
}
put方法繼承自hashmap,hashmap預留了 e.recordAccess(this)這個方法：
public V put(K key, V value) {
if (key == null)
return putForNullKey(value);
int hash = hash(key.hashCode());
int i = indexFor(hash, table.length);
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
addEntry(hash, key, value, i);
return null;
}
并通過重寫 addEntry(hash, key, value, i)這個方法，實現(xiàn)LRU中的刪除動作：
void addEntry(int hash, K key, V value, int bucketIndex) {
createEntry(hash, key, value, bucketIndex);
// Remove eldest entry if instructed, else grow capacity if appropriate
Entry<K,V> eldest = header.after;//找到最老的元素，這個在addBefore里確定，初次賦值是當只有一個head時候，你插入一個元素
if (removeEldestEntry(eldest)) {//這個是受保護的方法，需要自己制定刪除策略，比如size() > 最大容量,可自己實現(xiàn)，默認為false,也就是不開啟
removeEntryForKey(eldest.key);
} else {
if (size >= threshold)
resize(2 * table.length);
}
}
自己重寫這個方法，指定刪除策略：
protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
return false;
}
因此，可用linkedhashmap 構(gòu)建一個基于LRU算法的緩存。
package com.google.study.cache;
import java.util.LinkedHashMap;
import java.util.concurrent.locks.ReentrantLock;
public class SimpleCache<K, V> extends LinkedHashMap<K, V> {
private int maxCapacity;
private ReentrantLock lock = new ReentrantLock();
public SimpleCache(int maxCapacity, float load_factory) {
super(maxCapacity, load_factory, true);
this.maxCapacity = maxCapacity;
}
public int getMaxCapacity() {
return maxCapacity;
}
public void setMaxCapacity(int maxCapacity) {
this.maxCapacity = maxCapacity;
}
@Override
protected boolean removeEldestEntry(java.util.Map.Entry<K, V> eldest) {
// TODO Auto-generated method stub
return super.removeEldestEntry(eldest);
}
public V get(Object key) {
lock.lock();
try {
return super.get(key);
} finally {
lock.unlock();
}
}
public V put(K k, V v) {
lock.lock();
try {
return super.put(k, v);
} finally {
lock.unlock();
}
}
@Override
public void clear() {
lock.lock();
try {
super.clear();
} finally {
lock.unlock();
}
}
@Override
public int size() {
lock.lock();
try {
return super.size();
} finally {
lock.unlock();
}
}
}