In this article, we will discuss how to get synchronized version of Map using Collections class’s utility synchronizedMap() method
Q) How to make Synchronized Map ?
- From original collection framework introduced in Java 1.2 version, by default HashMap, LinkedHashMap and TreeMap classes aren’t thread-safe i.e.; non-synchronized
- But we can make it thread-safe using Collections utility synchronizedMap(map) method
- While iterating synchronized Map, make sure to iterate inside synchronized block;
- otherwise we may face non-deterministic behavior
From Java doc,
* It is imperative that the user manually synchronize on the returned
* map when iterating over any of its collection views:
Map m = Collections.synchronizedMap(new HashMap());
...
Set s = m.keySet(); // Needn't be in synchronized block
...
synchronized (m) { // Synchronizing on m, not s!
Iterator i = s.iterator(); // Must be in synchronized block
while (i.hasNext())
foo(i.next());
}
* Failure to follow this advice may result in non-deterministic behavior.
1. To get Synchronized version of Map
Method signature:
public static Map synchronizedMap(Map<Object> map);
SynchronizedVersionOfMap.java
package in.bench.resources.java.collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
public class SynchronizedVersionOfMap {
public static void main(String[] args) {
// creating HashMap object of type String, String
HashMap<String, String> unSynchronizedMap =
new HashMap<String, String>();
// adding elements to HashMap object
unSynchronizedMap.put("Facebook", "Mark Zuckerberg");
unSynchronizedMap.put("LinkedIn", "Reid Hoffman");
unSynchronizedMap.put("YouTube", " Steve Chen");
unSynchronizedMap.put("Google", "Larry Page");
unSynchronizedMap.put("Google", "Sergey Brin"); // duplicate
unSynchronizedMap.put("Twitter", "Jack Dorsey");
// to get synchronized HashMap
Map<String, String> synchronizedMap = Collections
.synchronizedMap(unSynchronizedMap);
// get entries from HashMap
Set<Entry,<String, String>> set = synchronizedMap.entrySet();
System.out.println("Iterating through synchronized HashMap\n");
// iterate inside synchronized block
synchronized(synchronizedMap) {
Iterator<Entry<String, String>> iterator = set.iterator();
System.out.println("Company\t\tFounder Name");
System.out.println("========\t=================");
while (iterator.hasNext()) {
Map.Entry<String, String> mapEntry =
(Map.Entry<String, String>) iterator.next();
System.out.println(mapEntry.getKey()
+ "\t\t" + mapEntry.getValue());
}
}
}
}
Output:
Iterating through synchronized HashMap
Company Founder Name
======== =================
Google Sergey Brin
Twitter Jack Dorsey
LinkedIn Reid Hoffman
Facebook Mark Zuckerberg
YouTube Steve Chen
Note: similarly, we can make thread-safe for LinkedHashMap or TreeMap classes
Related Articles:
- Java – How to get synchronized version of List ?
- Java – How to get synchronized version of Set ?
- Java – How to get synchronized version of Map ?
- Java – How to get synchronized version Collection ?
References:
- https://docs.oracle.com/javase/tutorial/collections/intro/
- https://docs.oracle.com/javase/tutorial/collections/interfaces/collection.html
- https://docs.oracle.com/javase/7/docs/api/java/util/Collection.html
- https://docs.oracle.com/javase/tutorial/collections/interfaces/order.html
- https://docs.oracle.com/javase/7/docs/api/java/util/Collections.html
- https://docs.oracle.com/javase/7/docs/api/java/util/Map.html
- https://docs.oracle.com/javase/7/docs/api/java/util/Map.Entry.html
- https://docs.oracle.com/javase/tutorial/collections/interfaces/map.html
- https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html
Happy Coding !!
Happy Learning !!
