Sync

Class Overview

Main interface for locks, gates, and conditions.

Sync objects isolate waiting and notification for particular logical states, resource availability, events, and the like that are shared across multiple threads. Use of Syncs sometimes (but by no means always) adds flexibility and efficiency compared to the use of plain java monitor methods and locking, and are sometimes (but by no means always) simpler to program with.

Most Syncs are intended to be used primarily (although not exclusively) in before/after constructions such as:

 class X {
     Sync gate;
 
     // ...
 
    public void m() { 
      try {
        gate.acquire();  // block until condition holds
        try {
          // ... method body
        }
        finally {
          gate.release()
        }
      }
      catch (InterruptedException ex) {
        // ... evasive action
      }
    }
    
    public void m2(Sync cond) { // use supplied condition
      try {
        if (cond.attempt(10)) {         // try the condition for 10 ms
          try {
            // ... method body
          }
          finally {
            cond.release()
          }
        }
      }
      catch (InterruptedException ex) {
        // ... evasive action
      }
    }
  }
 
 

Syncs may be used in somewhat tedious but more flexible replacements for built-in Java synchronized blocks. For example:

 class HandSynched {
     private double state_ = 0.0;
 
     private final Sync lock; // use lock type supplied in constructor
 
     public HandSynched(Sync l) {
         lock = l;
     }
 
     public void changeState(double d) {
         try {
             lock.acquire();
             try {
                 state_ = updateFunction(d);
             } finally {
                 lock.release();
             }
         } catch (InterruptedException ex) {
         }
     }
 
     public double getState() {
         double d = 0.0;
         try {
             lock.acquire();
             try {
                 d = accessFunction(state_);
             } finally {
                 lock.release();
             }
         } catch (InterruptedException ex) {
         }
         return d;
     }
 
 private double updateFunction(double d) { ... }private double accessFunction(double d) { ... }}
 
 

If you have a lot of such methods, and they take a common form, you can standardize this using wrappers. Some of these wrappers are standardized in LockedExecutor, but you can make others. For example:

 class HandSynchedV2 {
     private double state_ = 0.0;
 
     private final Sync lock; // use lock type supplied in constructor
 
     public HandSynchedV2(Sync l) {
         lock = l;
     }
 
     protected void runSafely(Runnable r) {
         try {
             lock.acquire();
             try {
                 r.run();
             } finally {
                 lock.release();
             }
         } catch (InterruptedException ex) { // propagate without throwing
             Thread.currentThread().interrupt();
         }
     }
 
     public void changeState(double d) {
         runSafely(new Runnable() {
             public void run() {
                 state_ = updateFunction(d);
             }
         });
     }
     // ...
 }
 

One reason to bother with such constructions is to use deadlock- avoiding back-offs when dealing with locks involving multiple objects. For example, here is a Cell class that uses attempt to back-off and retry if two Cells are trying to swap values with each other at the same time.

 
  class Cell {
    long value;
    Sync lock = ... // some sync implementation class
    void swapValue(Cell other) {
      for (;;) { 
        try {
          lock.acquire();
          try {
            if (other.lock.attempt(100)) {
              try { 
                long t = value; 
                value = other.value;
                other.value = t;
                return;
              }
              finally { other.lock.release(); }
            }
          }
          finally { lock.release(); }
        } 
        catch (InterruptedException ex) { return; }
      }
    }
  }
 
 

Here is an even fancier version, that uses lock re-ordering upon conflict:

 
  class Cell { 
    long value;
    Sync lock = ...;
    private static boolean trySwap(Cell a, Cell b) {
      a.lock.acquire();
      try {
        if (!b.lock.attempt(0)) 
          return false;
        try { 
          long t = a.value;
          a.value = b.value;
          b.value = t;
          return true;
        }
        finally { other.lock.release(); }
      }
      finally { lock.release(); }
      return false;
    }
 
   void swapValue(Cell other) {
     try {
       while (!trySwap(this, other) &&
             !tryswap(other, this)) 
         Thread.sleep(1);
     }
     catch (InterruptedException ex) { return; }
   }
 }
 
 

Interruptions are in general handled as early as possible. Normally, InterruptionExceptions are thrown in acquire and attempt(msec) if interruption is detected upon entry to the method, as well as in any later context surrounding waits. However, interruption status is ignored in release();

Timed versions of attempt report failure via return value. If so desired, you can transform such constructions to use exception throws via

 if (!c.attempt(timeval))
     throw new TimeoutException(timeval);
 

The TimoutSync wrapper class can be used to automate such usages.

All time values are expressed in milliseconds as longs, which have a maximum value of Long.MAX_VALUE, or almost 300,000 centuries. It is not known whether JVMs actually deal correctly with such extreme values. For convenience, some useful time values are defined as static constants.

All implementations of the three Sync methods guarantee to somehow employ Java synchronized methods or blocks, and so entail the memory operations described in JLS chapter 17 which ensure that variables are loaded and flushed within before/after constructions.

Syncs may also be used in spinlock constructions. Although it is normally best to just use acquire(), various forms of busy waits can be implemented. For a simple example (but one that would probably never be preferable to using acquire()):

 
  class X {
    Sync lock = ...
    void spinUntilAcquired() throws InterruptedException {
      // Two phase. 
      // First spin without pausing.
      int purespins = 10; 
      for (int i = 0; i < purespins; ++i) {
        if (lock.attempt(0))
          return true;
      }
      // Second phase - use timed waits
      long waitTime = 1; // 1 millisecond
      for (;;) {
        if (lock.attempt(waitTime))
          return true;
        else 
          waitTime = waitTime * 3 / 2 + 1; // increase 50% 
      }
    }
  }
  
 

In addition pure synchronization control, Syncs may be useful in any context requiring before/after methods. For example, you can use an ObservableSync (perhaps as part of a LayeredSync) in order to obtain callbacks before and after each method invocation for a given class.

[ Introduction to this package. ]

Summary

Constants
long	ONE_CENTURY	One century in milliseconds; convenient as a time-out value *
long	ONE_DAY	One day, in milliseconds; convenient as a time-out value *
long	ONE_HOUR	One hour, in milliseconds; convenient as a time-out value *
long	ONE_MINUTE	One minute, in milliseconds; convenient as a time-out value *
long	ONE_SECOND	One second, in milliseconds; convenient as a time-out value *
long	ONE_WEEK	One week, in milliseconds; convenient as a time-out value *
long	ONE_YEAR	One year in milliseconds; convenient as a time-out value *

Public Methods
abstract void	acquire() Wait (possibly forever) until successful passage.
abstract boolean	attempt(long msecs) Wait at most msecs to pass; report whether passed.
abstract void	release() Potentially enable others to pass.