JAVA並發編程學習筆記------鎖順序死結

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一、需求描述：

　　將資金從一個賬戶轉移到另一個賬戶。

二、程式實現：

（1）賬戶類：

public class Account {    private long account;    public Account(String user, long account) {        this.account = account;    }    public Account() {        super();    }    public long getAccount() {        return account;    }    public void setAccount(long account) {        this.account = account;    }    public void debit(long money){        this.account -= money;    }    public void credit(long money){        this.account += money;    }}

　　（2）資產轉移類：

public class TransMoney {    private static final Object tieLock = new Object();    public static void transferMoney(Account fromAccount,Account toAccount,long amount){        synchronized (fromAccount){            synchronized (toAccount){                fromAccount.debit(amount);                toAccount.credit(amount);            }        }    }}

　　（3）測試類別：

public class DemonstrateDeadLock {    private static final int NUM_THREADS = 20;    private static final int NUM_ACCOUNTS = 5;    private static final int NUM_ITERATIONS = 1000000;    public static void main(String[] args) {        final Random rdn = new Random();        final Account[] accounts = new Account[NUM_ACCOUNTS];        for(int i=0;i<accounts.length;i++){            accounts[i] = new Account();        }        class TransferThread extends Thread{            public void run(){                for(int i=0;i<NUM_ITERATIONS;i++){                    int fromAccount = rdn.nextInt(NUM_ACCOUNTS);                    int toAccount = rdn.nextInt(NUM_ACCOUNTS);                    TransMoney.transferMoney(accounts[fromAccount],accounts[toAccount],rdn.nextInt(1000));                }            }        }        for(int i=0;i<NUM_THREADS;i++){            new TransferThread().start();        }    }}

　　（4）解析：

上述程式容易形成死結，原因在於多賬戶調用TransMoney.transferMoney時，存在鎖順序衝突，

解決方案是使用System.identityHashCode來定義鎖的順序，消除死結的可能性，代碼實現如下：

public static void transferMoney(final Account fromAccount,final Account toAccount,final long amount){    class Helper{        public void transfer(){            fromAccount.debit(amount);            toAccount.credit(amount);        }    }    int fromHash = System.identityHashCode(fromAccount);    int toHash = System.identityHashCode(toAccount);    if(fromHash < toHash){        synchronized (fromAccount){            synchronized (toAccount){                new Helper().transfer();            }        }    }else if(fromHash > toHash){        synchronized (toAccount){            synchronized (fromAccount){                new Helper().transfer();            }        }    }else{        synchronized (tieLock){            synchronized (fromAccount){                synchronized (toAccount){                    new Helper().transfer();                }            }        }    }}

 

JAVA並發編程學習筆記------鎖順序死結