Methods to aid classes recover from OutOfMemoryErrors by denying or reducing service rather than a complete shutdown of the JVM.
/*
Derby - Class org.apache.derby.iapi.services.memory.LowMemory
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*/
/**
* Methods to aid classes recover from OutOfMemoryErrors by denying
* or reducing service rather than a complete shutdown of the JVM.
* It's intended that classes use to functionality to allow then to
* deny service when memory is low to allow the JVM to recover,
* rather than start new operations that are probably doomed to
* failure due to the low memory.
* <P>
* Expected usage is one instance of this class per major logical
* operation, e.g. creating a connection, preparing a statement,
* adding an entry to a specific cache etc.
* <BR>
* The logical operation would call isLowMemory() before starting
* the operation, and thrown a static exception if it returns true.
* <BR>
* If during the operation an OutOfMemoryException is thrown the
* operation would call setLowMemory() and throw its static exception
* representing low memory.
* <P>
* Future enhancments could be a callback mechanism for modules
* where they register they can reduce memory usage on a low
* memory situation. These callbacks would be triggered by
* a call to setLowMemory. For example the page cache could
* reduce its current size by 10% in a low memory situation.
*
*/
public class LowMemory {
/**
* Free memory seen when caller indicated an out of
* memory situation. Becomes a low memory watermark
* for five seconds that causes isLowMemory to return
* true if free memory is lower than this value.
* This allows the JVM a chance to recover memory
* rather than start new operations that are probably
* doomed to failure due to the low memory.
*
*/
private long lowMemory;
/**
* Time in ms corresponding to System.currentTimeMillis() when
* lowMemory was set.
*/
private long whenLowMemorySet;
/**
* Set a low memory watermark where the owner of this object just hit an
* OutOfMemoryError. The caller is assumed it has just freed up any
* references it obtained during the operation, so that the freeMemory call
* as best as it can reflects the memory before the action that caused the
* OutOfMemoryError, not part way through the action.
*
*/
public void setLowMemory() {
// Can read lowMemory unsynchronized, worst
// case is that we force extra garbage collection.
if (lowMemory == 0L) {
// The caller tried to dereference any objects it
// created during its instantation. Try to garbage
// collect these so that we can a best-guess effort
// at the free memory before the overall operation we are
// failing on occurred. Of course in active multi-threading
// systems we run the risk that some other thread just freed
// up some memory that throws off our calcuation. This is
// avoided by clearing lowMemory some time later on an
// isLowMemory() call.
for (int i = 0; i < 5; i++) {
System.gc();
System.runFinalization();
try {
Thread.sleep(50L);
} catch (InterruptedException e) {
}
}
}
synchronized (this) {
if (lowMemory == 0L) {
lowMemory = Runtime.getRuntime().freeMemory();
whenLowMemorySet = System.currentTimeMillis();
}
}
}
/**
* Return true if a low memory water mark has been set and the current free
* memory is lower than it. Otherwise return false.
*/
public boolean isLowMemory() {
synchronized (this) {
long lm = lowMemory;
if (lm == 0)
return false;
if (Runtime.getRuntime().freeMemory() > lm)
return false;
// Only allow an low memory watermark to be valid
// for five seconds after it was set. This stops
// an incorrect limit being set for ever. This could
// occur if other threads were freeing memory when
// we called Runtime.getRuntime().freeMemory()
long now = System.currentTimeMillis();
if ((now - this.whenLowMemorySet) > 5000L) {
lowMemory = 0L;
whenLowMemorySet = 0L;
return false;
}
return true;
}
}
}
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