Why is hashmap not thread safe

Another point to note about Hashtable is that it does not allow null keys or values whereas HashMap allows one null key and any number of null values. SynchronizedMap is a static inner class of utility class java. It is quite similar to Hashtable and it also acquires lock on entire Map instance. It is not a legacy class like Hashtable and it was introduced in jdk 1. It provides functionality to convert any thread-unsafe Map implementation to thread-safe implementation using Collections.

For Example,. It internally wraps all the methods of Map interface with synchronized keyword. For example, here is the internal put , get and remove method implementation of SynchronizedMap :. Click here to check full code of SynchronizedMap. Similar to HashMap , Synchronized HashMap allows one null key and multiple null values as it just wraps the methods of HashMap with synchronized keyword.

Rest of the behavior remains same as original collection. You can also check SynchronizedSortedMap which is similar data structure to convert thread-unsafe TreeMap and other SortedMap implementation to corresponding thread-safe collection. Hashtable and SynchronizedMap both acquires lock on entire Map object which provides thread-safety, but not good performance as at a time only one thread can access that Map instance.

Concurrent package. More than one threads can read and write concurrently in ConcurrentHashMap and still it provides thread-safety. Amazing, isn't it? How is it implemented internally? Well, ConcurrentHashMap divides the Map instance into different segments. And each thread acquires lock on each segment. By default it allows 16 threads to access it simultaneously without any external synchronization i. We can also increase or decrease the default concurrency level while creating ConcurrentHashMap by using below constructor :.

Thread acquires a lock on segment in put operation and at a time only one thread can write in that segment. But without extra synchronization, it is a time-bomb waiting to go off. Thanks for visiting DZone today,. Edit Profile. Sign Out View Profile. Over 2 million developers have joined DZone. HashMap is not a Thread-Safe Structure. Like 0. We all know. HashMap It's not thread safe Asynchronous.

So how can we make HashMap Become thread safe? I think it can be realized mainly through the following three ways : 1. Replace with Hashtable,Hashtable Through the Thread safety is when multiple threads access , Lock mechanism is adopted , When a thread accesses some data of this class , To protect , Other threads can't access it until the thread finishes reading , Only other threads can use.

There will be no data inconsistency or data pollution. Thread insecurity means that it doesn't provide Hash Algorithm encryption : Create a nodejs file hash.