Java內(nèi)置鎖的簡(jiǎn)單認(rèn)識(shí)

2019-11-06 07:15:21

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供稿：網(wǎng)友

多線程開(kāi)發(fā)離不開(kāi)鎖機(jī)制，現(xiàn)在的java語(yǔ)言中，提供了2種鎖，一種是語(yǔ)言特性提供的內(nèi)置鎖，還有一種是 java.util.concurrent.locks 包中的鎖，這篇文章簡(jiǎn)單整理一下內(nèi)置鎖的知識(shí)點(diǎn)。

內(nèi)置鎖在Java語(yǔ)言中的表現(xiàn)：

多線程的鎖，其實(shí)本質(zhì)上就是給一塊內(nèi)存空間的訪問(wèn)添加訪問(wèn)權(quán)限，因?yàn)镴ava中是沒(méi)有辦法直接對(duì)某一塊內(nèi)存進(jìn)行操作的，又因?yàn)镴ava是面向?qū)ο蟮恼Z(yǔ)言，一切皆對(duì)象，所以具體的表現(xiàn)就是某一個(gè)對(duì)象承擔(dān)鎖的功能，每一個(gè)對(duì)象都可以是一個(gè)鎖。內(nèi)置鎖，使用方式就是使用 synchronized 關(guān)鍵字，synchronized 方法或者 synchronized 代碼塊。

每一種 synchronized 寫(xiě)法的鎖是哪個(gè)對(duì)象：

1、指定當(dāng)前對(duì)象加鎖：

    PRivate synchronized void function() {        //TODO execute something    } 
2、指定當(dāng)前類的Class對(duì)象加鎖：
    private static synchronized void function() {        //TODO execute something    }注意此處的 static 關(guān)鍵字。 
3、指定任意對(duì)象加鎖：
private void function() {    synchronized (object) {        //TODO execute something    }}此時(shí)，這段同步代碼塊的鎖加在object對(duì)象上面。該對(duì)象可以是當(dāng)前對(duì)象（object == this），也可以是當(dāng)前類的Class對(duì)象（object == MyClass.class）。
 
簡(jiǎn)單驗(yàn)證一下：
現(xiàn)有如下的類：
public class SynchronizedTest {    private Object lock = new Object();    public void synchronizedBlockOnObject(long executeTime) {        synchronized (lock) {            System.out.println(Thread.currentThread().getName() + " -> start synchronizedBlockOnObject");            doSomething(executeTime);            System.out.println(Thread.currentThread().getName() + " -> end synchronizedBlockOnObject");        }    }    public void synchronizedBlockOnThis(long executeTime) {        synchronized (this) {            System.out.println(Thread.currentThread().getName() + " -> start synchronizedBlockOnThis");            doSomething(executeTime);            System.out.println(Thread.currentThread().getName() + " -> end synchronizedBlockOnThis");        }    }    public void synchronizedBlockOnClass(long executeTime) {        synchronized (SynchronizedTest.class) {            System.out.println(Thread.currentThread().getName() + " -> start synchronizedBlockOnClass");            doSomething(executeTime);            System.out.println(Thread.currentThread().getName() + " -> end synchronizedBlockOnClass");        }    }    public synchronized void synchronizedMethodOnThis(long executeTime) {        System.out.println(Thread.currentThread().getName() + " -> start synchronizedMethodOnThis");        doSomething(executeTime);        System.out.println(Thread.currentThread().getName() + " -> end synchronizedMethodOnThis");    }    public static synchronized void synchronizedMethodOnClass(long executeTime) {        System.out.println(Thread.currentThread().getName() + " -> start synchronizedMethodOnClass");        doSomething(executeTime);        System.out.println(Thread.currentThread().getName() + " -> end synchronizedMethodOnClass");    }    private static void doSomething(long executeTime) {        try {            Thread.sleep(executeTime);        } catch (InterruptedException e) {            e.printStackTrace();        }    }} 
 
1、static synchronized 方法 和 synchronized (MyClass.class) {} 同步代碼塊的鎖都加在 MyClass.class 對(duì)象上面：
    public static void main(String[] args) {        SynchronizedTest synchronizedTest = new SynchronizedTest();                new Thread(new Runnable() {            @Override            public void run() {                SynchronizedTest.synchronizedMethodOnClass(3000);            }        }, "Thread static synchronized method").start();        new Thread(new Runnable() {            @Override            public void run() {                synchronizedTest.synchronizedBlockOnClass(2000);            }        }, "Thread synchronized block on Class").start();    }運(yùn)行結(jié)果如下：
Thread static synchronized method -> start synchronizedMethodOnClassThread static synchronized method -> end synchronizedMethodOnClassThread synchronized block on Class -> start synchronizedBlockOnClassThread synchronized block on Class -> end synchronizedBlockOnClass說(shuō)明當(dāng)線程 Thread static synchronized method 進(jìn)入方法 synchronizedMethodOnClass 的時(shí)候，線程Thread synchronized block on Class 是不能進(jìn)入synchronizedBlockOnClass 代碼塊的。
 
2、非 static 的 synchronized 方法和 synchronized (this) {} 同步代碼塊的鎖都加在當(dāng)前對(duì)象上面：
public static void main(String[] args) {    SynchronizedTest synchronizedTest = new SynchronizedTest();    new Thread(new Runnable() {        @Override        public void run() {            synchronizedTest.synchronizedMethodOnThis(3000);        }    }, "Thread non-static synchronized method").start();    new Thread(new Runnable() {        @Override        public void run() {            synchronizedTest.synchronizedBlockOnThis(2000);        }    }, "Thread synchronized block on this").start();}運(yùn)行結(jié)果如下：
Thread non-static synchronized method -> start synchronizedMethodOnThisThread non-static synchronized method -> end synchronizedMethodOnThisThread synchronized block on this -> start synchronizedBlockOnThisThread synchronized block on this -> end synchronizedBlockOnThis說(shuō)明當(dāng)線程 Thread non-static synchronized method 進(jìn)入方法 synchronizedMethodOnThis 的時(shí)候，線程Thread synchronized block on this 是不能進(jìn)入synchronizedBlockOnThis 代碼塊的。
 
3、當(dāng)鎖加在 MyClass.class 、 this 、 任意對(duì)象，這三種情況，起不到任何同步作用：
    public static void main(String[] args) {        SynchronizedTest synchronizedTest = new SynchronizedTest();        new Thread(new Runnable() {            @Override            public void run() {                synchronizedTest.synchronizedMethodOnThis(3000);            }        }, "Thread non-static synchronized method").start();        new Thread(new Runnable() {            @Override            public void run() {                SynchronizedTest.synchronizedMethodOnClass(2000);            }        }, "Thread static sybchronized method").start();        new Thread(new Runnable() {            @Override            public void run() {                synchronizedTest.synchronizedBlockOnObject(4000);            }        }, "Thread sybchronized block on other Object").start();    }運(yùn)行結(jié)果如下：
Thread non-static synchronized method -> start synchronizedMethodOnThisThread static sybchronized method -> start synchronizedMethodOnClassThread sybchronized block on other Object -> start synchronizedBlockOnObjectThread static sybchronized method -> end synchronizedMethodOnClassThread non-static synchronized method -> end synchronizedMethodOnThisThread sybchronized block on other Object -> end synchronizedBlockOnObject說(shuō)明當(dāng)鎖沒(méi)有加在同一個(gè)對(duì)象上的時(shí)候，起不到線程間的同步作用。
 
 
Object中對(duì)內(nèi)置鎖進(jìn)行操作的一些方法：
wait()系列：
wait()系列方法的作用是：使當(dāng)前已經(jīng)獲得該對(duì)象鎖的線程進(jìn)入等待狀態(tài)，并且釋放該對(duì)象的鎖。
notify()系列：
notify()系列方法的作用是：?jiǎn)拘涯切┱诘却搶?duì)象鎖的線程，使其繼續(xù)運(yùn)行。
 
基于wait() notify()機(jī)制，我們可以實(shí)現(xiàn)一個(gè)簡(jiǎn)易的生產(chǎn)者-消費(fèi)者模型。
大體思路如下，一個(gè)生產(chǎn)者線程負(fù)責(zé)向一個(gè)倉(cāng)庫(kù)中存放（put）物品，一個(gè)消費(fèi)者線程負(fù)責(zé)從倉(cāng)庫(kù)中取出（get）物品。
代碼如下：
public class Warehouse {    private Queue<Integer> queue;    private int capacity;    public Warehouse(int capacity) {        this.capacity = capacity;        queue = new LinkedList();    }    public synchronized void put(int num) {        if (queue.size() >= capacity) {            try {                System.out.println(Thread.currentThread().getName() + " , put full wait");                wait();            } catch (InterruptedException e) {                e.printStackTrace();            }        }        queue.add(num);        System.out.println(Thread.currentThread().getName() + " , put : " + num + "  , queue -> " + queue);        notifyAll();    }    public synchronized int get() {        if (queue.isEmpty()) {            try {                System.out.println(Thread.currentThread().getName() + " , get empty wait");                wait();            } catch (InterruptedException e) {                e.printStackTrace();            }        }        int num = queue.poll();        System.out.println(Thread.currentThread().getName() + " , get : " + num + "  , queue -> " + queue);        notifyAll();        return num;    }} 
    public static void main(String[] args) {        Warehouse warehouse = new Warehouse(4);        Random random = new Random();        new Thread(new Runnable() {            @Override            public void run() {                while (true) {                    warehouse.put(random.nextInt(10));                    try {                        Thread.sleep(1000);                    } catch (InterruptedException e) {                        e.printStackTrace();                    }                }            }        }, "生產(chǎn)者-01").start();        new Thread(new Runnable() {            @Override            public void run() {                while (true) {                    warehouse.get();                    try {                        Thread.sleep(2000);                    } catch (InterruptedException e) {                        e.printStackTrace();                    }                }            }        }, "消費(fèi)者-01").start();    }運(yùn)行結(jié)果如下：
生產(chǎn)者-01 , put : 5  , queue -> [5]消費(fèi)者-01 , get : 5  , queue -> []生產(chǎn)者-01 , put : 7  , queue -> [7]消費(fèi)者-01 , get : 7  , queue -> []生產(chǎn)者-01 , put : 9  , queue -> [9]生產(chǎn)者-01 , put : 7  , queue -> [9, 7]消費(fèi)者-01 , get : 9  , queue -> [7]生產(chǎn)者-01 , put : 0  , queue -> [7, 0]生產(chǎn)者-01 , put : 5  , queue -> [7, 0, 5]消費(fèi)者-01 , get : 7  , queue -> [0, 5]生產(chǎn)者-01 , put : 9  , queue -> [0, 5, 9]生產(chǎn)者-01 , put : 6  , queue -> [0, 5, 9, 6]消費(fèi)者-01 , get : 0  , queue -> [5, 9, 6]生產(chǎn)者-01 , put : 4  , queue -> [5, 9, 6, 4]生產(chǎn)者-01 , put full wait消費(fèi)者-01 , get : 5  , queue -> [9, 6, 4]生產(chǎn)者-01 , put : 6  , queue -> [9, 6, 4, 6]生產(chǎn)者-01 , put full wait消費(fèi)者-01 , get : 9  , queue -> [6, 4, 6]生產(chǎn)者-01 , put : 2  , queue -> [6, 4, 6, 2]生產(chǎn)者-01 , put full wait消費(fèi)者-01 , get : 6  , queue -> [4, 6, 2]生產(chǎn)者-01 , put : 9  , queue -> [4, 6, 2, 9]生產(chǎn)者-01 , put full wait消費(fèi)者-01 , get : 4  , queue -> [6, 2, 9]生產(chǎn)者-01 , put : 7  , queue -> [6, 2, 9, 7]生產(chǎn)者-01 , put full wait消費(fèi)者-01 , get : 6  , queue -> [2, 9, 7]生產(chǎn)者-01 , put : 2  , queue -> [2, 9, 7, 2]