hudson.util
Class KeyedDataStorage<T,P>

java.lang.Object
  hudson.util.KeyedDataStorage<T,P>

Direct Known Subclasses:

FingerprintMap

public abstract class KeyedDataStorage<T,P>
extends Object

Convenient base class for implementing data storage.

One typical pattern of data storage in Hudson is the one that Fingerprint uses, where each data is keyed by an unique key (MD5 sum), and that key is used to determine the file system location of the data. On memory, each data is represented by one object (Fingerprint), and write access to the same data is coordinated by using synchronization.

With such storage, care has to be taken to ensure that there's only one data object in memory for any given key. That means load and create operation needs to be synchronized. This class implements this logic in a fairly efficient way, and thus intends to help plugins that want to use such data storage.

Since:

1.87

Author:

Kohsuke Kawaguchi

See Also:

FingerprintMap

Field Summary

AtomicInteger	cacheHit Number of cache hits (of all the total queries.)
AtomicInteger	loadFailure Number of failures in loading data.
AtomicInteger	totalQuery Total number of queries into this storage.
AtomicInteger	weakRefLost Among cache misses, number of times when we had SoftReference but lost its value due to GC.

Constructor Summary

KeyedDataStorage()

Method Summary

protected abstract T	create(String key, P createParams) Creates a new data object.
T	get(String key) Finds the data object that matches the given key if available, or null if not found.
protected T	get(String key, boolean createIfNotExist, P createParams) Implementation of get/getOrCreate.
T	getOrCreate(String key, P createParams) Atomically gets the existing data object if any, or if it doesn't exist create it and return it.
String	getPerformanceStats() Gets the short summary of performance statistics.
protected abstract T	load(String key) Attempts to load an existing data object from disk.
void	resetPerformanceStats()

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

totalQuery

public final AtomicInteger totalQuery

Total number of queries into this storage.

cacheHit

public final AtomicInteger cacheHit

Number of cache hits (of all the total queries.)

weakRefLost

public final AtomicInteger weakRefLost

Among cache misses, number of times when we had SoftReference but lost its value due to GC. totalQuery-cacheHit-weakRefLost means cache miss.

loadFailure

public final AtomicInteger loadFailure

Number of failures in loading data.

Constructor Detail

KeyedDataStorage

public KeyedDataStorage()

Method Detail

getOrCreate

public T getOrCreate(String key,
                     P createParams)
              throws IOException

Atomically gets the existing data object if any, or if it doesn't exist create it and return it.

Parameters:

createParams - Additional parameters needed to create a new data object. Can be null.

Returns:

Never null.

Throws:

IOException

get

public T get(String key)
      throws IOException

Finds the data object that matches the given key if available, or null if not found.

Throws:

IOException

get

protected T get(String key,
                boolean createIfNotExist,
                P createParams)
         throws IOException

Implementation of get/getOrCreate.

Throws:

IOException

load

protected abstract T load(String key)
                   throws IOException

Attempts to load an existing data object from disk.

KeyedDataStorage class serializes the requests so that no two threads call the load(String) method with the same parameter concurrently. This ensures that there's only up to one data object for any key.

Returns:

null if no such data exists.

Throws:

IOException - if load operation fails. This exception will be propagated to the caller.

create

protected abstract T create(String key,
                            P createParams)
                     throws IOException

Creates a new data object.

This method is called by getOrCreate(String,Object) if the data that matches the specified key does not exist.

Because of concurrency, another thread might call get(String) as soon as a new data object is created, so it's important that this method returns a properly initialized "valid" object.

Returns:

never null. If construction fails, abort with an exception.

Throws:

IOException - if the method fails to create a new data object, it can throw IOException (or any other exception) and that will be propagated to the caller.

resetPerformanceStats

public void resetPerformanceStats()

getPerformanceStats

public String getPerformanceStats()

Gets the short summary of performance statistics.