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ReentrantLockOrder.java

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import java.util.concurrent.locks.ReentrantLock;

class ReentrantLockOrder {

private int primaryAccountBalance;

private final ReentrantLock primaryLock = new ReentrantLock();

private int savingsAccountBalance;

private final ReentrantLock savingsLock = new ReentrantLock();

public boolean transferToSavings(int amount) {

try {

primaryLock.lock();

savingsLock.lock();

if (amount>0 && primaryAccountBalance>=amount) {

primaryAccountBalance -= amount;

savingsAccountBalance += amount;

return true;

}

} finally {

savingsLock.unlock();

primaryLock.unlock();

}

return false;

}

public boolean transferToPrimary(int amount) {

// AVOID: lock order is different from "transferToSavings"

// and may result in deadlock

try {

savingsLock.lock();

primaryLock.lock();

if (amount>0 && primaryAccountBalance>=amount) {

primaryAccountBalance -= amount;

savingsAccountBalance += amount;

return true;

}

} finally {

primaryLock.unlock();

savingsLock.unlock();

}

return false;

}

}