Shared
Reentrant lock is useful, when you have a large number of resources that need
to be accessed concurrently by multiple threads, and you want to minimize
contention while allowing for parallel access to these resources.
ShardedReentrantLock.java
package com.sample.app.concurrency;
import java.util.concurrent.locks.ReentrantLock;
public class ShardedReentrantLock {
private final ReentrantLock locks[];
private final int size;
private final int mask;
public ShardedReentrantLock(int count) {
this.size = count;
mask = size - 1;
locks = new ReentrantLock[size];
for (int i = 0; i < size; i++) {
locks[i] = new ReentrantLock();
}
}
public ReentrantLock get(String key) {
return locks[key.hashCode() & mask];
}
}
Let's consider a scenario where you have a distributed cache that you want to implement using ShardedReentrantLock to ensure thread-safety and minimize contention when accessing cache entries. In this example, we'll create a simple cache using ShardedReentrantLock.
ShardedCache.java
package com.sample.app.concurrency;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReentrantLock;
public class ShardedCache<K, V> {
private final ShardedReentrantLock lock;
private final Map<K, V> cache;
public ShardedCache(int shardCount) {
this.lock = new ShardedReentrantLock(shardCount);
this.cache = new HashMap<>();
}
public V get(K key) {
ReentrantLock shardLock = lock.get(key.toString());
shardLock.lock();
try {
System.out.printf("%s read by %s\n", key, Thread.currentThread().getName());
return cache.get(key);
} finally {
shardLock.unlock();
}
}
public void put(K key, V value) {
ReentrantLock shardLock = lock.get(key.toString());
shardLock.lock();
try {
System.out.printf("%s added/updated by %s\n", key, Thread.currentThread().getName());
cache.put(key, value);
} finally {
shardLock.unlock();
}
}
public void remove(K key) {
ReentrantLock shardLock = lock.get(key.toString());
shardLock.lock();
try {
System.out.printf("%s removed by %s\n", key, Thread.currentThread().getName());
cache.remove(key);
} finally {
shardLock.unlock();
}
}
}
Let's test the application.
SharedCacheDemo.java
package com.sample.app;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import com.sample.app.concurrency.ShardedCache;
public class SharedCacheDemo {
private static final int SHARD_COUNT = 16;
private static final ShardedCache<String, String> SHARED_CACHE = new ShardedCache<>(SHARD_COUNT);
private static enum CacheTaskType {
READ, UPDATE, REMOVE
}
private static class CacheTask implements Runnable {
CacheTaskType cacheTaskType;
String key;
String value;
public CacheTask(CacheTaskType cacheTaskType, String key) {
this(cacheTaskType, key, null);
}
public CacheTask(CacheTaskType cacheTaskType, String key, String value) {
this.cacheTaskType = cacheTaskType;
this.key = key;
this.value = value;
}
@Override
public void run() {
if (CacheTaskType.READ == cacheTaskType) {
SHARED_CACHE.get(key);
return;
}
if (CacheTaskType.UPDATE == cacheTaskType) {
SHARED_CACHE.put(key, value);
return;
}
if (CacheTaskType.REMOVE == cacheTaskType) {
SHARED_CACHE.remove(key);
return;
}
}
}
public static void main(String[] args) {
ExecutorService executor = new ThreadPoolExecutor(SHARD_COUNT, SHARD_COUNT, 0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
for (int i = 0; i < 100; i++) {
String key = "key_" + i;
String value = "value_" + i;
if (i % 3 == 0) {
CacheTask cacheTask = new CacheTask(CacheTaskType.READ, key);
executor.execute(cacheTask);
} else if (i % 3 == 1) {
CacheTask cacheTask = new CacheTask(CacheTaskType.UPDATE, key, value);
executor.execute(cacheTask);
} else {
CacheTask cacheTask = new CacheTask(CacheTaskType.REMOVE, key);
executor.execute(cacheTask);
}
}
executor.shutdown();
}
}
Sample Output
key_0 read by pool-1-thread-1 key_10 added/updated by pool-1-thread-11 key_9 read by pool-1-thread-10 key_8 removed by pool-1-thread-9 key_7 added/updated by pool-1-thread-8 key_6 read by pool-1-thread-7 key_5 removed by pool-1-thread-6 key_4 added/updated by pool-1-thread-5 key_3 read by pool-1-thread-4 key_2 removed by pool-1-thread-3 key_1 added/updated by pool-1-thread-2 key_13 added/updated by pool-1-thread-14 key_14 removed by pool-1-thread-15 key_15 read by pool-1-thread-16 key_16 added/updated by pool-1-thread-1 key_17 removed by pool-1-thread-11 key_21 read by pool-1-thread-7 key_18 read by pool-1-thread-10 key_20 removed by pool-1-thread-8 key_19 added/updated by pool-1-thread-9 key_11 removed by pool-1-thread-12 key_31 added/updated by pool-1-thread-11 key_29 removed by pool-1-thread-16 key_32 removed by pool-1-thread-7 key_28 added/updated by pool-1-thread-15 key_27 read by pool-1-thread-14 key_30 read by pool-1-thread-1 key_26 removed by pool-1-thread-2 key_25 added/updated by pool-1-thread-3 key_24 read by pool-1-thread-4 key_12 read by pool-1-thread-13 key_35 removed by pool-1-thread-9 key_36 read by pool-1-thread-12 key_43 added/updated by pool-1-thread-2 key_37 added/updated by pool-1-thread-11 key_41 removed by pool-1-thread-14 key_42 read by pool-1-thread-1 key_38 removed by pool-1-thread-16 key_40 added/updated by pool-1-thread-15 key_39 read by pool-1-thread-7 key_22 added/updated by pool-1-thread-6 key_51 read by pool-1-thread-14 key_54 read by pool-1-thread-15 key_49 added/updated by pool-1-thread-2 key_53 removed by pool-1-thread-16 key_52 added/updated by pool-1-thread-1 key_50 removed by pool-1-thread-11 key_48 read by pool-1-thread-12 key_47 removed by pool-1-thread-9 key_46 added/updated by pool-1-thread-13 key_23 removed by pool-1-thread-5 key_57 read by pool-1-thread-14 key_65 removed by pool-1-thread-13 key_58 added/updated by pool-1-thread-15 key_64 added/updated by pool-1-thread-9 key_63 read by pool-1-thread-12 key_59 removed by pool-1-thread-2 key_62 removed by pool-1-thread-11 key_61 added/updated by pool-1-thread-1 key_74 removed by pool-1-thread-1 key_60 read by pool-1-thread-16 key_33 read by pool-1-thread-10 key_76 added/updated by pool-1-thread-16 key_71 removed by pool-1-thread-12 key_75 read by pool-1-thread-1 key_72 read by pool-1-thread-2 key_73 added/updated by pool-1-thread-11 key_70 added/updated by pool-1-thread-9 key_69 read by pool-1-thread-15 key_68 removed by pool-1-thread-13 key_34 added/updated by pool-1-thread-8 key_85 added/updated by pool-1-thread-13 key_79 added/updated by pool-1-thread-12 key_84 read by pool-1-thread-15 key_83 removed by pool-1-thread-9 key_81 read by pool-1-thread-2 key_82 added/updated by pool-1-thread-11 key_80 removed by pool-1-thread-1 key_77 removed by pool-1-thread-10 key_91 added/updated by pool-1-thread-2 key_93 read by pool-1-thread-1 key_92 removed by pool-1-thread-11 key_90 read by pool-1-thread-9 key_87 read by pool-1-thread-13 key_86 removed by pool-1-thread-8 key_45 read by pool-1-thread-4 key_97 added/updated by pool-1-thread-11 key_98 removed by pool-1-thread-9 key_96 read by pool-1-thread-1 key_95 removed by pool-1-thread-2 key_94 added/updated by pool-1-thread-10 key_44 removed by pool-1-thread-3 key_56 removed by pool-1-thread-6 key_55 added/updated by pool-1-thread-7 key_67 added/updated by pool-1-thread-14 key_66 read by pool-1-thread-5 key_78 read by pool-1-thread-16 key_88 added/updated by pool-1-thread-12 key_89 removed by pool-1-thread-15 key_99 read by pool-1-thread-13
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