【JAVA並發編程實戰】8、鎖順序死結，java實戰

【JAVA並發編程實戰】8、鎖順序死結，java實戰

package cn.study.concurrency.ch10;public class Account {    private String staffAccount;    //帳號    private String passWord;    //密碼    private int balance; //賬戶餘額        public Account(int money) {        this.balance = money;    }        public String getStaffAccount() {        return staffAccount;    }    public void setStaffAccount(String staffAccount) {        this.staffAccount = staffAccount;    }    public String getPassWord() {        return passWord;    }    public void setPassWord(String passWord) {        this.passWord = passWord;    }        public void debit(int amount)    {        System.out.println("轉出賬戶：" + amount);    }        public void credit(int amount)    {        System.out.println("轉入賬戶：" + amount);    }    public int getBalance() {        return balance;    }    public void setBalance(int balance) {        this.balance = balance;    }    }

 

package cn.study.concurrency.ch10;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import javax.naming.InsufficientResourcesException;/** * 通過制定確定的鎖順序來避免死結 * @author xiaof * */public class DeathLock {    public void transferMoney(Account fromAccount, Account toAccount, int amount) throws InsufficientResourcesException    {        synchronized(fromAccount)        {            synchronized(toAccount)            {                //按參數的順序上鎖，這個依據參數的調用方法的順序                if(fromAccount.getBalance() < amount)                {                    //賬戶餘額不足，無法轉賬                    throw new InsufficientResourcesException();                }                else                {                    fromAccount.debit(amount);                    toAccount.credit(amount);                }            }        }    }    /**     * 這個用來在無法判定枷鎖順序的時候的加時賽鎖     */    private static final Object tieLock = new Object();        public static void transferMoney2(final Account fromAccount, final Account toAccount, final int amount) throws InsufficientResourcesException    {        /**         * 輔助內部類         * @author xiaof         *         */        class Helper        {            public void transfer() throws InsufficientResourcesException            {                //內部類可以隨意訪問外部類成員                //按參數的順序上鎖，這個依據參數的調用方法的順序                if(fromAccount.getBalance() < amount)                {                    //賬戶餘額不足，無法轉賬                    throw new InsufficientResourcesException();                }                else                {                    fromAccount.debit(amount);                    toAccount.credit(amount);                }            }        }        //返回給定對象的雜湊碼，該代碼與預設的方法 hashCode() 返回的代碼一樣，無論給定對象的類是否重寫 hashCode()        int fromHash = System.identityHashCode(fromAccount);        int toHash = System.identityHashCode(toAccount);        //根據hash值判定加鎖順序，那麼一樣的對象的鎖順序就一定一樣        if(fromHash < toHash)        {            synchronized(fromAccount)            {                synchronized(toAccount)                {                    new Helper().transfer();                }            }        }        else if(toHash < fromHash)        {            synchronized(toAccount)            {                synchronized(fromAccount)                {                    new Helper().transfer();                }            }        }        else        {            //如果很不巧，hash值是一樣的，那麼就需要一個加時賽的機制，先擷取外部鎖，然後再此基礎上對兩個對象隨機上鎖            synchronized(tieLock)            {                synchronized(fromAccount)                {                    synchronized(toAccount)                    {                        new Helper().transfer();                    }                }            }        }            }        static Account account1 = new Account(999);    static Account account2 = new Account(999);        public static void main(String[] args) throws InsufficientResourcesException {        //對於第一個方法很容易死結        //比如：當有兩個同時執行這個方法的調用時候//        DeathLock dl = new DeathLock();        //這個時候第一個調用在鎖了account1，然後第二個調用鎖了account2        //同時第一個需要account2，第二個需要account1，這就發生競爭死結了//        dl.transferMoney(account1, account2, 998);//        dl.transferMoney(account2, account1, 998);//        //        dl.transferMoney2(account1, account2, 998);        ExecutorService pool = Executors.newFixedThreadPool(10);        for(int i = 0; i < 5; ++ i)        {            pool.execute(new Runnable() {                @Override                public void run() {                    try {                        DeathLock.transferMoney2(account1, account2, 998);                    } catch (InsufficientResourcesException e) {                        e.printStackTrace();                    }                }            });        }                for(int i = 0; i < 5; ++ i)        {            pool.execute(new Runnable() {                @Override                public void run() {                    try {                        DeathLock.transferMoney2(account2, account1, 998);                    } catch (InsufficientResourcesException e) {                        e.printStackTrace();                    }                }            });        }                pool.shutdown();    }}

測試結果：