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Introduction to ConcurrentHashMap in Java


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ConcurrentHashMap is a subclass of HashMap and is designed for use in multi-threaded environments. It maintains a configurable concurrency stage (default worth of 16), which could be specified whereas creating the map. The upper the concurrency stage, the extra threads can entry its buckets without delay, however this can lead to extra competition if all getEnumerators() are usually not synchronized appropriately. This Java programming tutorial discusses ConcurrentHashMap in Java with code examples wherever applicable.

To be taught extra about hashing, try our tutorial: Introduction to Hashing in Java.

What Is ConcurrentHashMap in Java?

In Java, a ConcurrentHashMap is an information construction used for storing and retrieving information in a concurrent method. This information construction is designed for use in multi-threaded functions the place a number of threads are studying and writing to the hash map concurrently.

ConcurrentHashMap is a Java class that allows a number of threads to entry the map on the identical level of time. It is part of the Java Collections Framework, and it supplies a manner for functions to effectively deal with concurrent updates to the map.

The ConcurrentHashMap class supplies a number of strategies for doing primary operations on the map, corresponding to placing values into the map and getting values out of the map. Moreover, the category supplies a number of strategies for doing extra superior operations, corresponding to iterating over the key-value pairs within the map or retrieving a set of keys which might be related to a given worth.

Why Ought to Builders Use ConcurrentHashMap?

The ConcurrentHashMap is a strong information construction that allows Java builders to work with concurrent information in a secure and environment friendly method. The ConcurrentHashMap supplies plenty of options that makes it effectively suited to working with concurrent information, together with:

  • Atomic operations: All operations on the ConcurrentHashMap are atomic, which suggests you’ll be able to safely carry out concurrent information entry.
  • Lock-free: The ConcurrentHashMap is designed to be lock-free, which implies that there isn’t any want to amass a lock so as to learn or write information. This makes the ConcurrentHashMap very environment friendly when utilized in a multi-threaded atmosphere.
  • Scalable: The ConcurrentHashMap is designed to be scalable, which suggests it could effectively deal with massive quantities of information.

Learn: The Finest Instruments for Distant Builders

How Does ConcurrentHashMap Work in Java?

ConcurrentHashMap is a strong information construction that allows quick, thread-safe operations on information. The map works by permitting every thread to securely entry and modify the info with out having to fret about different threads interfering. That is completed by utilizing a particular locking mechanism that solely permits one thread to entry the info at a time.

This locking mechanism is named a segmented lock. It really works by dividing the info into small segments, after which permitting every thread to lock one section at a time. This manner, a number of threads can work on the info concurrently, however they may by no means be engaged on the identical information on the identical time.

The segmented lock is carried out utilizing a particular Java class referred to as ReentrantLock. ReentrantLock is a strong synchronization device that permits threads to securely lock information buildings. It’s utilized by many Java lessons, together with ConcurrentHashMap.

The ReentrantLock class has two important strategies: lock() and unlock(). When a thread calls lock(), it’ll try to amass the lock for the info construction. If the lock is offered, the thread will purchase it and may then safely entry and modify the info.

Within the absence of a lock, the thread will wait till one other thread releases it. As soon as the thread has acquired the lock, it could execute the crucial part code after which launch it by calling the unlock() technique.

The ReentrantLock class additionally has a tryLock() technique, which permits a thread to aim to amass the lock with out ready. This may be helpful in conditions the place you do not need to dam different threads if the lock just isn’t out there.

Tips on how to Program ConcurrentHashMap in Java

The ConcurrentHashMap class extends from the HashMap class and it’s carried out to help concurrent entry. It’s also thread-safe, which implies that a number of threads can entry it with none points. To create a ConcurrentHashMap, you’ll be able to write the next Java code:

ConcurrentHashMap concurrentHashMap = new ConcurrentHashMap();  

The next code itemizing can be utilized to retailer gadgets in a ConcurrentHashMap after which show them on the console window:

import java.util.concurrent.*;
class MyConcurrentHashMap {
 public static void important(String[] args)
    {
        ConcurrentHashMap<String, String> concurrentHashMap
            = new ConcurrentHashMap<>();
 
        concurrentHashMap.put("P001", "Lenovo Legion");
        concurrentHashMap.put("P002", "DELL Inspiron");
        concurrentHashMap.put("P003", "Lenovo ThinkPad");
        System.out.println(concurrentHashMap);
    }
}

When to Use ConcurrentHashMap in Java?

When a number of threads entry an information construction on the identical level of time, it could result in unpredictable outcomes and even deadlocks. Which means if you’ll implement your individual model, you have to make it possible for it’s thread-safe by utilizing synchronization primitives corresponding to locks or semaphores.

ConcurrentHashMap is a good alternative for high-performance functions, because it gives quick lookup and insertion instances. It gives a number of options that make it well-suited for this sort of atmosphere, together with the power to lock and unlock particular person buckets, in addition to to atomically add and take away components.

Mainly, you must use it everytime you want a concurrent map that doesn’t block when used. For instance, if you happen to’re working with loads of thread-safe information, or if you have to entry the map from a number of threads, ConcurrentHashMap is an effective choice. It’s additionally a good selection if you have to retailer a considerable amount of information and need it to be accessible from a number of threads.

The ConcurrentHashMap is designed to be accessed by a number of threads for studying or writing information. You’ll be able to entry it from a number of threads with out locking the whole object so that you don’t have to attend when one thread locks one other a part of your program whereas accessing it.

ConcurrentHashMap additionally helps atomic putIfAbsent() and get() operations which lets you lock solely that merchandise if it exists so as to add new information or retrieve current information from map respectively with none deadlocks if different threads are already operating these operations on another entry inside this map.

When utilizing a Hashtable or synchronized assortment, the whole assortment is locked throughout reads and writes. With a ConcurrentHashMap, solely the required ingredient is locked for studying or writing. This enables for higher efficiency as a result of multiple thread can learn information without delay.

Don’t use it if you happen to want sequential entry to components within the map. A concurrent map doesn’t assure that an merchandise might be out there instantly after one other merchandise has been modified; there might be different threads modifying totally different components of it at any given time cut-off date which means that there isn’t any assured ordering after modifications.

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ConcurrentHashMap vs HashMap in Java

At the beginning, the ConcurrentHashMap class is designed to be used in concurrent information entry, whereas the Hashtable class just isn’t. Which means the ConcurrentHashMap class could be safely utilized by a number of threads concurrently, with out the danger of information corruption or race circumstances.

Second, the ConcurrentHashMap class is rather more environment friendly than the Hashtable class. It makes use of a hash-based algorithm to retailer and retrieve information, which makes it quicker and extra scalable. Lastly, the ConcurrentHashMap class gives plenty of highly effective options that aren’t out there within the Hashtable class. These embody help for locking, evicting and iterating over entries, and including and eradicating entries from the map.

There are a number of totally different situations wherein it is smart to make use of a ConcurrentHashMap over an everyday HashMap. One widespread state of affairs is when you have to help a number of readers and writers concurrently. This might be the case in an online utility, for instance, the place a number of customers are accessing information on the identical time.

One other state of affairs wherein ConcurrentHashMap is helpful is when you have to carry out frequent reads and rare writes. On this case, it may be extra environment friendly to make use of a ConcurrentHashMap as a result of writes won’t block reads.

ConcurrentHashMap is good for high-performance, multi-threaded functions. If you have to entry and replace a map from a number of threads, ConcurrentHashMap is the best choice. It supplies all of the operations of a HashMap and moreover permits concurrent entry for learn, write, and replace.

ConcurrentHashMap vs Hashtable in Java

ConcurrentHashMap makes use of a number of locks, so totally different components of the map could be up to date concurrently with out blocking one another. Hashtable, then again, makes use of a single lock for the whole map, which may result in efficiency points. Nonetheless, in contrast to Hashtable, accesses to ConcurrentHashMap are typically not blocked when a lock is unavailable. This enables for a lot greater concurrency in retrieving information from a ConcurrentHashMap.

When updating a ConcurrentHashMap, a number of threads can lock totally different parts of the map for updates. The variety of locks is configurable at creation time, and could be adjusted if wanted.

Locking a portion of the map solely blocks different threads from updating the locked portion; it doesn’t block different threads from studying different components of the map that aren’t locked. Not like a Hashtable, null keys and values are usually not allowed in a ConcurrentHashMap.

When To not use ConcurrentHashMap in Java

ConcurrentHashMap just isn’t a good selection if you happen to would want to take care of insertion order of the weather. It’s because the inner construction of a ConcurrentHashMap is predicated on hash codes, which implies that the order wherein components are inserted just isn’t preserved.

One other scenario wherein it isn’t preferrred to make use of a ConcurrentHashMap is when you have to synchronize entry to the whole map. It’s because every section of the map is locked independently, so locking the whole map would require locking every section individually, which might be inefficient.

Remaining Ideas on ConcurrentHashMap in Java

ConcurrentHashMap is a Java class that permits a number of threads to entry the map concurrently. It is part of the Java Collections Framework, and it supplies a manner for functions to effectively deal with concurrent updates to the map. ConcurrentHashMap is totally different from conventional HashMap in that it makes use of a lock to make sure that just one thread can modify the map at a time. This prevents information corruption and race circumstances.

ConcurrentHashMap is an extension of the Map interface that may maintain a number of entries on the identical time. You’ll be able to benefit from ConcurrentHashMap to carry out concurrent updates on its assortment of keys, and values with out locking out readers or writers for any important time period. The important thing options of ConcurrentHashMap are: efficiency, scalability and thread security, that are crucial for concurrent programming.

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