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To avoid data corruption in multithreaded Java programs, shared data must be protected from concurrent modifications and accesses. This Locking can be performed at the object level by using synchronized methods or , synchronized blocks, or by using the java.util.concurrent dynamic lock objects. However, excessive use of locking may can result in deadlocks (See CON08-J. Do not call alien methods that synchronize on the same objects as any callers in the execution chain). .

Java neither prevents deadlocks nor requires their detection [JLS 2015]. Deadlock can occur when two or more threads request and release locks in different orders. Consequently, programs are required to avoid deadlock by acquiring and releasing locks in the same order.

Additionally, To avoid deadlock, locks should be acquired and released in the same order and synchronization should be limited to cases where it is absolutely necessary. For instanceexample, to avoid deadlocks, the paint(), dispose(), stop(), and destroy() methods should never be synchronized in an applet should not be synchronized because they are always called and used from dedicated threads.

"The Java programming language neither prevents nor requires detection of deadlock conditions." \[[JLS 05|AA. Java References#JLS 05]\]. Deadlocks can arise when two or more threads request and release locks in different orders. 

Noncompliant Code Example

. Furthermore, the Thread.stop() and Thread.destroy() methods are deprecated (see THI05-J. Do not use Thread.stop() to terminate threads for more information).

This rule also applies to programs that need to work with a limited set of resources. For example, liveness issues can arise when two or more threads are waiting for each other to release resources such as database connections. These issues can be resolved by letting each waiting thread retry the operation at random intervals until they successfully acquire the resource.

Noncompliant Code Example (Different Lock Orders)

This noncompliant code example can deadlock because of excessive synchronization. The balanceAmount field represents the total balance available for a particular BankAccount object. Users are allowed to initiate an operation that atomically transfers a specified amount from one account to anotherThis noncompliant code example can deadlock because of excessive synchronization. Assume that an attacker has two bank accounts and is capable of requesting two depositAllAmount() operations in succession, one each from the two threads started in main().

final class BankAccount {
  private intdouble balanceAmount;  // Total amount in bank account
	 
  private BankAccount(intdouble balance) {
    this.balanceAmount = balance;
  }

  // Deposits the amount from this object instance
  // to BankAccount instance argument ba 
  private void depositAllAmountdepositAmount(BankAccount ba, double amount) {
    synchronized (this) {
      synchronized (ba) {
        ba.balanceAmount += this.balanceAmount;if (amount > balanceAmount) {
        this.balanceAmount = 0;throw // Withdraw all amount from this instance
  new IllegalArgumentException(
           ba.displayAllAmount();  // Display the"Transfer newcannot balanceAmountbe incompleted"
 ba (may cause deadlock)
      });
    }  
  }
  
   private synchronized void displayAllAmount() { ba.balanceAmount += amount;
    System.out.println(balanceAmount);    this.balanceAmount -= amount;
      }
    }
  }

  public static void initiateTransfer(final BankAccount first,
    final BankAccount second, final double amount) {

    Thread ttransfer = new Thread(new Runnable() {
        public void run() {
          first.depositAllAmountdepositAmount(second, amount);
        }
    });
    ttransfer.start();
  }
}

Objects of class BankAccount represent bank accounts. The balanceAmount field represents the total balance amount available for a particular object (bank account). A user is allowed to initiate an operation deposit all amount that transfers the balance amount from one account to another. This is equivalent to closing a bank account and transferring the balance to a different (existing or new) account.

Objects of this class are deadlock-prone. An attacker may cause the program to construct two threads that initiate balance transfers from two different BankAccount object instances, a and b. Consider the following code that does this:

this class are prone to deadlock. An attacker who has two bank accounts can construct two threads that initiate balance transfers from two different BankAccount object instances a and b. For example, consider the following code:

BankAccount a = new BankAccount(5000);
BankAccount b = new BankAccount(6000);
BankAccount.initiateTransfer(a, b, 1000); // starts thread 1
BankAccount.initiateTransfer(b, a, 1000); // starts thread 2

The two transfers are Each transfer is performed in their own threads, from instance a to b and b to aits own thread. The first thread atomically transfers the amount from a to b by depositing the balance from a to it in account b and then withdrawing the entire balance same amount from a. The second thread performs the reverse operation, ; that is, it transfers the balance amount from b to a and withdraws the balance from b. When executing depositAllAmountdepositAmount(), the first thread might acquire acquires a lock on object a while the . The second thread may could acquire a lock on object b before the first thread can. Subsequently, the first thread requests would request a lock on b, which is already held by the second thread and the . The second thread requests would request a lock on a, which is already held by the first thread. This constitutes a deadlock condition , as because neither thread can proceed.

The threads in this program request monitors in different orders This noncompliant code example may or may not deadlock, depending on the interleaving of method calls. If Thread T1 finishes executing before Thread T2, or T2 before T1, there are no issues because in these cases, locks are acquired and released in the same order. Sequences where the threads alternate, such as, T1, T2, T1, T2 may cause a deadlock.

Compliant Solution (static internal private lock)

scheduling details of the platform. Deadlock occurs when (1) two threads request the same two locks in different orders, and (2) each thread obtains a lock that prevents the other thread from completing its transfer. Deadlock is avoided when two threads request the same two locks but one thread completes its transfer before the other thread begins. Similarly, deadlock is avoided if the two threads request the same two locks in the same order (which would happen if they both transfer money from one account to a second account) or if two transfers involving distinct accounts occur concurrently.

Compliant Solution (Private Static Final Lock Object)

This compliant solution avoids deadlock by synchronizing on a private static final The deadlock can be avoided by using a private static final internal lock object before performing any account transfers.:

final class BankAccount {
  private intdouble balanceAmount;  // Total amount in bank account
	 
  private static final Object lock = new Object();

  private BankAccount(intdouble balance) {
    this.balanceAmount = balance;
    this.lock = new Object();
  }

  // Deposits the amount from this object instance
  // to BankAccount instance argument ba 
  private void depositAllAmountdepositAmount(BankAccount ba, double amount) {
    synchronized (lock) {
      ba.balanceAmount += this.balanceAmount;if (amount > balanceAmount) {
      this.balanceAmount = 0;throw // Withdraw all amount from this instance
new IllegalArgumentException(
           ba.displayAllAmount();  // Display the new balanceAmount in ba (may cause deadlock) "Transfer cannot be completed");
    } 
  }
  
  private void displayAllAmount() {
    synchronized (lock) { ba.balanceAmount += amount;
      System.outthis.println(balanceAmount)balanceAmount -= amount;
    }
  }

  public static void initiateTransfer(final BankAccount first,
    final BankAccount second), {
final double amount) {

    Thread ttransfer = new Thread(new Runnable() {
        @Override public void run() {
          first.depositAllAmountdepositAmount(second, amount);
        }
    });
    ttransfer.start();
  }
}

In this scenario, if deadlock cannot occur when two threads with two different BankAccount objects try to transfer to each othersother's accounts simultaneously, deadlock cannot occur. One thread will acquire the private lock, complete its transfer, and release the lock , before the other thread can proceed.

This solution comes with imposes a performance penalty because a private static lock restricts the system to performing only one transfer at a timetransfers sequentially. Two transfers involving four distinct accounts (with distinct target accounts) may not happen cannot be performed concurrently. The impact of this This penalty increases considerably as the number of BankAccount objects increase. Consequently, this solution does not fails to scale very well.

Compliant Solution (

...

Ordered Locks)

This compliant solution ensures that multiple locks are acquired and released in the same order. It requires that an a consistent ordering over BankAccount objects is available. The ordering is enforced by having the class BankAccount extend . Consequently, the BankAccount class implements the java.lang.Comparable interface and overriding overrides the compareTo() method.

final class BankAccount implements ComparableComparable<BankAccount> {
  private intdouble balanceAmount;  // Total amount in bank account	 
  private final Object lock;

  private final BankAccount(int balance) {
    this.balanceAmount = balance;
    this.locklong id; // Unique for each BankAccount
  private static final AtomicLong nextID = new ObjectAtomicLong(0);
 // Next unused }ID

  // Deposits the amount from this object instance to BankAccount instance argument ba 
  private void depositAllAmount(BankAccount ba) {BankAccount(double balance) {
    this.balanceAmount = balance;
    this.lock = new Object();
    BankAccount former, latterthis.id = nextID.getAndIncrement();
  }

  if@Override public int (compareTo(BankAccount ba) < 0) {
     return former = this;
      latter = ba;
    } else {
      former = ba;
      latter = this;
    }
    synchronized (former) {
      synchronized (latter(this.id > ba.id) ? 1 : (this.id < ba.id) ? -1 : 0;
  }

  // Deposits the amount from this object instance
  // to BankAccount instance argument ba
  public void depositAmount(BankAccount ba, double amount) {
    BankAccount former, latter;
    if (compareTo(ba) < 0) {
        ba.balanceAmountformer += this.balanceAmount;
       latter this.balanceAmount = 0ba;
 // withdraw all amount} fromelse this instance{
      former = ba.displayAllAmount();
 // Display the new balanceAmount inlatter ba (may cause deadlock)= this;
    }
  } 
 synchronized (former)  }{
  }
 
  private synchronized void displayAllAmount(latter) {
        if System.out.println((amount > balanceAmount); {
  }

  public static void initiateTransfer(final BankAccount first, finalthrow BankAccount second) {new IllegalArgumentException(
    Thread t = new Thread(new Runnable() {         "Transfer cannot be completed");
      public void run() {}
        first.depositAllAmount(second)ba.balanceAmount += amount;
      }  this.balanceAmount -= amount;
      });
    t.start();}
  }

  public intstatic compareTo(BankAccount ba) {void initiateTransfer(final BankAccount first,
   if(this.balanceAmount < ba.balanceAmount final BankAccount second, final double amount) {

    Thread return -1;
   } else if(this.balanceAmount > ba.balanceAmounttransfer = new Thread(new Runnable() {
     return  1;
 @Override public }void elserun() {
     return 0;
    first.depositAmount(second, amount);
        }
    });
    transfer.start();
  }
}

Whenever a transfer occurs, the two BankAccount objects are ordered so that the first object's lock is acquired before the second object's lock. Consequently, if When two threads attempt transfers between the same two accounts, they will both each try to acquire the first account's lock before acquiring the second account's lock. Consequently, with the result that one thread will acquire acquires both locks, complete completes the transfer, and release releases both locks before the other thread may can proceed.

Unlike the previous compliant solution, this solution incurs no performance penalty because permits multiple concurrent transfers can occur concurrently as long as the transfers involve distinct target accounts.

Compliant Solution (ReentrantLock)

In this compliant solution, each BankAccount has a java.util.concurrent.locks.ReentrantLock associated with it. . This design permits the depositAllAmountdepositAmount() method to try acquiring attempt to acquire the locks of both accounts' locks, but releasing to release the locks if it fails, and trying to try again later if necessary.

final class BankAccount {
  private intdouble balanceAmount;  // Total amount in bank account
  private final Lock lock = new ReentrantLock();
  private static final intRandom TIMEnumber = 1000; // 1 second
	 
  private new Random(123L);

  BankAccount(intdouble balance) {
    this.balanceAmount = balance;
  }

  // Deposits the amount from this object instance
  // to BankAccount instance argument ba 
  private void depositAllAmountdepositAmount(BankAccount ba, double amount)
 throws                             throws InterruptedException {
    while (true) {
      if (this.lock.tryLock()) {
        try {
          if (ba.lock.tryLock()) {
            try {
              ba.balanceAmount += this.balanceAmount;if (amount > balanceAmount) {
              this.balanceAmount = 0;throw // withdraw all amount from this instance
new IllegalArgumentException(
                   ba.displayAllAmount();  // Display the new balanceAmount in ba  "Transfer cannot be completed");
              }
              ba.balanceAmount += amount;
              this.balanceAmount -= amount;
              break;
            } finally {
              ba.lock.unlock();
            }
          }
        } finally {
          this.lock.unlock();
        }
      }
      Thread.sleep(TIMEint n = number.nextInt(1000);
    }
  }
int TIME 
= 1000 private+ void displayAllAmount() throws InterruptedException {
    while (true) {n; // 1 second + random delay to prevent livelock
      if (lock.tryLock()) {Thread.sleep(TIME);
    }
  }

  trypublic {
static void initiateTransfer(final        System.out.println(balanceAmount);BankAccount first,
    final BankAccount second, final double amount) break;{

    Thread transfer = new } finallyThread(new Runnable() {
        public void lock.unlockrun(); {
        }  try {
      }
      Threadfirst.sleep(TIMEdepositAmount(second, amount);
    }
  }

  public static void} catch initiateTransfer(finalInterruptedException BankAccount first, final BankAccount second) {
e) {
        Thread t = new Thread(new Runnable() {
.currentThread().interrupt(); // Reset interrupted status
       public void run() {}
        try {}
    });
      firsttransfer.depositAllAmountstart(second);
        } catch (InterruptedException e) {
          // Forward to handler
        }
      }
    });
    t.start();
  }
}
}
}

Deadlock is impossible in this compliant solution because no method grabs a lock and holds it locks are never held indefinitely. If the current object's lock is acquired , but the the second lock is unavailable, the first lock is released and the thread sleeps for some specified amount of time before retryingattempting to reacquire the lock.

Code that uses this lock behaves locking strategy has behavior similar to that of synchronized code that uses the traditional monitor lock. ReentrantLock also provides several other capabilities. For example, for instance, the tryLock() method does not block waiting if immediately returns false when another thread is already holding holds the lock. The class Further, the java.util.concurrent.locks.ReentrantReadWriteLock can be used when some thread requires class has multiple-readers/single-writer semantics and is useful when some threads require a lock to write information while other threads require the lock to concurrently read the information.

Noncompliant Code Example

...

(Different Lock Orders, Recursive)

The following immutable WebRequest class Consider an immutable class WebRequest that encapsulates a web request to received by a server.:

// Immutable WebRequest
public final class WebRequest {
  private final long bandwidth;
  private final long responseTime;

  public WebRequest(long getBandwidth(bandwidth, long responseTime) {
    returnthis.bandwidth = bandwidth;
    this.responseTime = responseTime;
  }

  public long getResponseTimegetBandwidth() {
    return responseTimebandwidth;
  }
 
  public WebRequest(long bandwidth, long responseTimegetResponseTime() {
    this.bandwidth = bandwidthreturn responseTime;
    this.responseTime = responseTime;
  }
}
}

Each request has a response time associated with it, as well as along with a measurement of the network bandwidth required to fulfill the request.

This noncompliant code example consists of an application that monitors web requests . It calculates and provides routines for calculating the average bandwidth and average response time required to service all serve incoming requests.

public final class WebRequestAnalyzer {
  private final Vector<WebRequest> requests = new Vector<WebRequest>();
  
  public boolean addWebRequest(WebRequest request) {
    // Lock on last element to prevent data race with the calculateAverageResponseTime() method
return requests.add(new WebRequest(request.getBandwidth(),
              synchronized (requests.lastElement()) {
      // Defensive copying
      return requests.add(new WebRequest(request.getBandwidth(), request.getResponseTime()));
    }
  }
  
  public double getAverageBandwidth() { 
    return calculateAverageBandwidth(0,if (requests.size() == 0); {
  }

  public double getAverageResponseTime()throw { new IllegalStateException("The vector is empty!");
    }
    return calculateAverageResponseTime(requests.size() - 1calculateAverageBandwidth(0, 0);
  }

  privatepublic double calculateAverageBandwidth(int i, long bandwidthgetAverageResponseTime() { 
    if (i > requests.size() == 0) {
       return bandwidth / requests.size(throw new IllegalStateException("The vector is empty!");
    }
    synchronizedreturn calculateAverageResponseTime(requests.elementAtsize(i)) {
      bandwidth += requests.get(i).getBandwidth();
      return calculateAverageBandwidth(++i, bandwidth); // Acquires locks in increasing order 
    }- 1, 0);
  }

  private double calculateAverageResponseTimecalculateAverageBandwidth(int i, long responseTimebandwidth) { 
    if (i <= -1= requests.size()) {		 
      return responseTimebandwidth / requests.size();
    }     
    synchronized (requests.elementAt(i)) {
      responseTimebandwidth += requests.get(i).getResponseTimegetBandwidth();
      return calculateAverageResponseTime(--i, responseTime); // Acquires locks in decreasingincreasing order
      return calculateAverageBandwidth(++i, bandwidth);
    }
  }
}

The monitoring application is built upon class WebRequestAnalyzer that maintains a list of web requests using vector requests. The vector requests is suitably initialized after defensively copying the requests. Any thread can get the average bandwidth or average response time of all web requests by invoking the getAverageBandwidth() and getAverageResponseTime() methods.

These methods use fine-grained locking by holding locks on individual elements (requests) of the vector. The locks permit new requests to be added while the computations are still underway. Consequently, the statistics reported by the methods are accurate at the time they return the results.

Unfortunately, this implementation is prone to deadlock because the recursive calls within the synchronized regions of these methods acquire the intrinsic locks in opposite numerical orders. That is, calculateAverageBandwidth() requests locks from index 0 to requests.size() - 1 whereas calculateAverageResponseTime() requests them from index requests.size() - 1 to 0. Because of recursion, no previously acquired locks are released by either method. A deadlock occurs when two threads call these methods out of order in that, one thread calls calculateAverageBandwidth() while the other calls calculateAverageResponseTime() before either method has finished executing.

For example, if there are 20 requests in the vector, and one thread calls getAverageBandwidth(), it acquires the intrinsic lock of WebRequest 0, the first element in the vector. Meanwhile, if a second thread calls getAverageResponseTime(), it acquires the intrinsic lock for WebRequest 19, the last element in the vector. Consequently, deadlock results because neither thread can acquire all of the locks and proceed with the calculations.

Compliant Solution

In this compliant solution, the two calculation methods acquire and release locks in the same order.

public class WebRequestAnalyzer {
  private final Vector<WebRequest> requests = new Vector<WebRequest>();
  
  public boolean addWebRequest(WebRequest request) {
    // No need to lock on the last element because locks are acquired in increasing order
    return requests.add(new WebRequest(request.getBandwidth(), request.getResponseTime()));  
  }

  public double getAverageBandwidth() { 
    return calculateAverageBandwidth(0, 0);
  }

  public double getAverageResponseTime() { 
    return calculateAverageResponseTime(0, 0);
  }

  private double calculateAverageBandwidth(int i, long bandwidth) { 
    if (i > requests.size()) {
      return bandwidth / requests.size();
    }
    synchronized (requests.elementAt(i)) { // Acquires locks in increasing order
      bandwidth += requests.get(i).getBandwidth();
      return calculateAverageBandwidth(++i, bandwidth);  
    }
  }

  private double calculateAverageResponseTime(int i, long responseTime) { 
    if (i > requests.size()) {		 
      return responseTime / requests.size();
    }     
    synchronized (requests.elementAt(i)) {
      responseTime += requests.get(i).getResponseTime();
      return calculateAverageResponseTime(++i, responseTime); // Acquires locks in increasing order
    }
  }
}

  private double calculateAverageResponseTime(int i, long responseTime) {
    if (i <= -1) {
      return responseTime / requests.size();
    }
    synchronized (requests.elementAt(i)) {
      responseTime += requests.get(i).getResponseTime();
      // Acquires locks in decreasing order
      return calculateAverageResponseTime(--i, responseTime);
    }
  }
}

The monitoring application is built around the WebRequestAnalyzer class, which maintains a list of web requests using the requests vector and includes the addWebRequest() setter method. Any thread can request the average bandwidth or average response time of all web requests by invoking the getAverageBandwidth() and getAverageResponseTime() methods.

These methods use fine-grained locking by holding locks on individual elements (web requests) of the vector. These locks permit new requests to be added while the computations are still underway. Consequently, the statistics reported by the methods are accurate when they return the results.

Unfortunately, this noncompliant code example is prone to deadlock because the recursive calls within the synchronized regions of these methods acquire the intrinsic locks in opposite numerical orders. That is, calculateAverageBandwidth() requests locks from index 0 up to requests.size() − 1, whereas calculateAverageResponseTime() requests them from index requests.size() − 1 down to 0. Because of recursion, previously acquired locks are never released by either method. Deadlock occurs when two threads call these methods out of order, because one thread calls calculateAverageBandwidth(), while the other calls calculateAverageResponseTime() before either method has finished executing.

For example, when there are 20 requests in the vector, and one thread calls getAverageBandwidth(), the thread acquires the intrinsic lock of WebRequest 0, the first element in the vector. Meanwhile, if a second thread calls getAverageResponseTime(), it acquires the intrinsic lock of WebRequest 19, the last element in the vector. Consequently, deadlock results because neither thread can acquire all of the locks required to proceed with its calculations.

Note that the addWebRequest() method also has a race condition with calculateAverageResponseTime(). While iterating over the vector, new elements can be added to the vector, invalidating the results of the previous computation. This race condition can be prevented by locking on the last element of the vector (when it contains at least one element) before inserting the element.

Compliant Solution

In this compliant solution, the two calculation methods acquire and release locks in the same order, beginning with the first web request in the vector.

public final class WebRequestAnalyzer {
  private final Vector<WebRequest> requests = new Vector<WebRequest>();

  public boolean addWebRequest(WebRequest request) {
    return requests.add(new WebRequest(request.getBandwidth(),
                        request.getResponseTime()));
  }

  public double getAverageBandwidth() {
    if (requests.size() == 0) {
      throw new IllegalStateException("The vector is empty!");
    }
    return calculateAverageBandwidth(0, 0);
  }

  public double getAverageResponseTime() {
    if (requests.size() == 0) {
      throw new IllegalStateException("The vector is empty!");
    }
    return calculateAverageResponseTime(0, 0);
  }

  private double calculateAverageBandwidth(int i, long bandwidth) {
    if (i == requests.size()) {
      return bandwidth / requests.size();
    }
    synchronized (requests.elementAt(i)) {
      // Acquires locks in increasing order
      bandwidth += requests.get(i).getBandwidth();
      return calculateAverageBandwidth(++i, bandwidth);
    }
  }

  private double calculateAverageResponseTime(int i, long responseTime) {
    if (i == requests.size()) {
      return responseTime / requests.size();
    }
    synchronized (requests.elementAt(i)) {
      // Acquires locks in increasing order
      responseTime += requests.get(i).getResponseTime();
      return calculateAverageResponseTime(++i, responseTime);
    }
  }
}

Consequently, while Consequently, while one thread is calculating the average bandwidth or response time, another thread cannot interfere or induce a deadlock. This is because the other thread would first have to synchronize on the first WebRequest, which is impossible until the first calculation is completeor induce deadlock. Each thread must first synchronize on the first web request, which cannot happen until any prior calculation completes.

Locking on the last element of the vector in addWebRequest() is unnecessary for two reasons. First, the locks are acquired in increasing order in all the methods. Second, updates to the vector are reflected in the results of the computations.

Risk Assessment

Acquiring and releasing locks in the wrong order may can result in deadlocks deadlock.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[JLS 05|AA. Java References#JLS 05]\] [Chapter 17, Threads and Locks|http://java.sun.com/docs/books/jls/third_edition/html/memory.html]
\[[Halloway 00|AA. Java References#Halloway 00]\] 
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 412|http://cwe.mitre.org/data/definitions/412.html] "Unrestricted Lock on Critical Resource"

Automated Detection

Some static analysis tools can detect violations of this rule.

Related Guidelines

Bibliography

...

Image Added Image Added Image AddedCON11-J. Do not assume that declaring an object volatile guarantees visibility of its members      11. Concurrency (CON)      CON13-J. Do not try to force thread shutdown