Java 7: HashMap vs ConcurrentHashMap

Sep. 07, 12 · Interview

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As you may have seen from my past performance related articles and HashMap case studies, Java thread safety problems can bring down your Java EE application and the Java EE container fairly easily. One of most common problems I have observed when troubleshooting Java EE performance problems is infinite looping triggered from the non-thread safe HashMap get() and put() operations. This problem is known since several years but recent production problems have forced me to revisit this issue one more time.

This article will revisit this classic thread safety problem and demonstrate, using a simple Java program, the risk associated with a wrong usage of the plain old java.util.HashMap data structure involved in a concurrent threads context.

This proof of concept exercise will attempt to achieve the following 3 goals:

Revisit and compare the Java program performance level between the non-thread safe and thread safe Map data structure implementations (HashMap, Hashtable, synchronized HashMap, ConcurrentHashMap)
Replicate and demonstrate the HashMap infinite looping problem using a simple Java program that everybody can compile, run and understand
Review the usage of the above Map data structures in a real-life and modern Java EE container implementation such as JBoss AS7

For more detail on the ConcurrentHashMap implementation strategy, I highly recommend the great article from Brian Goetz on this subject.

Tools and server specifications

As a starting point, find below the different tools and software’s used for the exercise:

Sun/Oracle JDK & JRE 1.7 64-bit
Eclipse Java EE IDE
Windows Process Explorer (CPU per Java Thread correlation)
JVM Thread Dump (stuck thread analysis and CPU per Thread correlation)

The following local computer was used for the problem replication process and performance measurements:

Intel(R) Core(TM) i5-2520M CPU @ 2.50Ghz (2 CPU cores, 4 logical cores)
8 GB RAM
Windows 7 64-bit

* Results and performance of the Java program may vary depending of your workstation or server specifications.

Java program

In order to help us achieve the above goals, a simple Java program was created as per below:

The main Java program is HashMapInfiniteLoopSimulator.java
A worker Thread class WorkerThread.java was also created

The program is performing the following:

Initialize different static Map data structures with initial size of 2
Assign the chosen Map to the worker threads (you can chose between 4 Map implementations)
Create a certain number of worker threads (as per the header configuration). 3 worker threads were created for this proof of concept NB_THREADS = 3;
Each of these worker threads has the same task: lookup and insert a new element in the assigned Map data structure using a random Integer element between 1 – 1 000 000.
Each worker thread perform this task for a total of 500K iterations
The overall program performs 50 iterations in order to allow enough ramp up time for the HotSpot JVM
The concurrent threads context is achieved using the JDK ExecutorService

As you can see, the Java program task is fairly simple but complex enough to generate the following critical criteria’s:

Generate concurrency against a shared / static Map data structure
Use a mix of get() and put() operations in order to attempt to trigger internal locks and / or internal corruption (for the non-thread safe implementation)
Use a small Map initial size of 2, forcing the internal HashMap to trigger an internal rehash/resize

Finally, the following parameters can be modified at your convenience:

## Number of worker threads
private static final int NB_THREADS = 3;

## Number of Java program iterations
private static final int NB_TEST_ITERATIONS = 50;

## Map data structure assignment. You can choose between 4 structures
// Plain old HashMap (since JDK 1.2)
nonThreadSafeMap = new HashMap<String, Integer>(2);

// Plain old Hashtable (since JDK 1.0)
threadSafeMap1 = new Hashtable<String, Integer>(2);

// Fully synchronized HashMap
threadSafeMap2 = new HashMap<String, Integer>(2);
threadSafeMap2 = Collections.synchronizedMap(threadSafeMap2);

// ConcurrentHashMap (since JDK 1.5)
threadSafeMap3 = new ConcurrentHashMap<String, Integer>(2);

/*** Assign map at your convenience ****/
assignedMapForTest = threadSafeMap3;

Now find below the source code of our sample program.

#### HashMapInfiniteLoopSimulator.java
package org.ph.javaee.training4;

import java.util.Collections;
import java.util.Map;
import java.util.HashMap;
import java.util.Hashtable;

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 * HashMapInfiniteLoopSimulator
 * @author Pierre-Hugues Charbonneau
 *
 */
public class HashMapInfiniteLoopSimulator {

       private static final int NB_THREADS = 3;
       private static final int NB_TEST_ITERATIONS = 50;

       private static Map<String, Integer> assignedMapForTest = null;
       private static Map<String, Integer> nonThreadSafeMap = null;
       private static Map<String, Integer> threadSafeMap1 = null;
       private static Map<String, Integer> threadSafeMap2 = null;
       private static Map<String, Integer> threadSafeMap3 = null;

       /**
        * Main program
        * @param args
        */
       public static void main(String[] args) {

             System.out.println("Infinite Looping HashMap Simulator");
             System.out.println("Author: Pierre-Hugues Charbonneau");
             System.out.println("http://javaeesupportpatterns.blogspot.com");

             for (int i=0; i<NB_TEST_ITERATIONS; i++) {

                    // Plain old HashMap (since JDK 1.2)
                    nonThreadSafeMap = new HashMap<String, Integer>(2);

                    // Plain old Hashtable (since JDK 1.0)
                    threadSafeMap1 = new Hashtable<String, Integer>(2);

                    // Fully synchronized HashMap
                    threadSafeMap2 = new HashMap<String, Integer>(2);
                    threadSafeMap2 = Collections.synchronizedMap(threadSafeMap2);

                    // ConcurrentHashMap (since JDK 1.5)
                    threadSafeMap3 = new ConcurrentHashMap<String, Integer>(2); // ConcurrentHashMap


                    /*** Assign map at your convenience ****/
                    assignedMapForTest = threadSafeMap3;


                    long timeBefore = System.currentTimeMillis();
                    long timeAfter = 0;
                    Float totalProcessingTime = null;

                    ExecutorService executor = Executors.newFixedThreadPool(NB_THREADS);

                    for (int j = 0; j < NB_THREADS; j++) {

                           /** Assign the Map at your convenience **/
                           Runnable worker = new WorkerThread(assignedMapForTest);
                           executor.execute(worker);              
                    }

                    // This will make the executor accept no new threads
                    // and finish all existing threads in the queue
                    executor.shutdown();
                    // Wait until all threads are finish
                    while (!executor.isTerminated()) {

                    }

                    timeAfter = System.currentTimeMillis();
                    totalProcessingTime = new Float( (float) (timeAfter - timeBefore) / (float) 1000);

                    System.out.println("All threads completed in "+totalProcessingTime+" seconds");
             }
       }

}

#### WorkerThread.java
package org.ph.javaee.training4;

import java.util.Map;

/**
 * WorkerThread
 *
 * @author Pierre-Hugues Charbonneau
 *
 */
public class WorkerThread implements Runnable {

       private Map<String, Integer> map = null;

       public WorkerThread(Map<String, Integer> assignedMap) {
             this.map = assignedMap;

       }

       @Override
       public void run() {

             for (int i=0; i<500000; i++) {
                    // Return 2 integers between 1-1000000 inclusive
                    Integer newInteger1 = (int) Math.ceil(Math.random() * 1000000);
                    Integer newInteger2 = (int) Math.ceil(Math.random() * 1000000);                    

                    // 1. Attempt to retrieve a random Integer element
                    Integer retrievedInteger = map.get(String.valueOf(newInteger1));

                    // 2. Attempt to insert a random Integer element
                    map.put(String.valueOf(newInteger2), newInteger2);                
             }
       }

}

Performance comparison between thread safe Map implementations

The first goal is to compare the performance level of our program when using different thread safe Map implementations:

Plain old Hashtable (since JDK 1.0)
Fully synchronized HashMap (via Collections.synchronizedMap())
ConcurrentHashMap (since JDK 1.5)

Find below the graphical results of the execution of the Java program for each iteration along with a sample of the program console output.

# Output when using ConcurrentHashMap

Infinite Looping HashMap Simulator
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com
All threads completed in 0.984 seconds
All threads completed in 0.908 seconds
All threads completed in 0.706 seconds
All threads completed in 1.068 seconds
All threads completed in 0.621 seconds
All threads completed in 0.594 seconds
All threads completed in 0.569 seconds
All threads completed in 0.599 seconds
………………

As you can see, the ConcurrentHashMap is the clear winner here, taking in average only half a second (after an initial ramp-up) for all 3 worker threads to concurrently read and insert data within a 500K looping statement against the assigned shared Map. Please note that no problem was found with the program execution e.g. no hang situation.

The performance boost is definitely due to the improved ConcurrentHashMap performance such as the non-blocking get() operation.

The 2 other Map implementations performance level was fairly similar with a small advantage for the synchronized HashMap.

HashMap infinite looping problem replication

The next objective is to replicate the HashMap infinite looping problem observed so often from Java EE production environments. In order to do that, you simply need to assign the non-thread safe HashMap implementation as per code snippet below:

/*** Assign map at your convenience ****/
assignedMapForTest = nonThreadSafeMap;

Running the program as is using the non-thread safe HashMap should lead to:

No output other than the program header
Significant CPU increase observed from the system
At some point the Java program will hang and you will be forced to kill the Java process

What happened? In order to understand this situation and confirm the problem, we will perform a CPU per Thread analysis from the Windows OS using Process Explorer and JVM Thread Dump.

1 - Run the program again then quickly capture the thread per CPU data from Process Explorer as per below. Under explore.exe you will need to right click over the javaw.exe and select properties. The threads tab will be displayed. We can see overall 4 threads using almost all the CPU of our system.

2 – Now you have to quickly capture a JVM Thread Dump using the JDK 1.7 jstack utility. For our example, we can see our 3 worker threads which seems busy/stuck performing get() and put() operations.

..\jdk1.7.0\bin>jstack 272
2012-08-29 14:07:26
Full thread dump Java HotSpot(TM) 64-Bit Server VM (21.0-b17 mixed mode):

"pool-1-thread-3" prio=6 tid=0x0000000006a3c000 nid=0x18a0 runnable [0x0000000007ebe000]
   java.lang.Thread.State: RUNNABLE
        at java.util.HashMap.put(Unknown Source)
        at org.ph.javaee.training4.WorkerThread.run(WorkerThread.java:32)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)

"pool-1-thread-2" prio=6 tid=0x0000000006a3b800 nid=0x6d4 runnable [0x000000000805f000]
   java.lang.Thread.State: RUNNABLE
        at java.util.HashMap.get(Unknown Source)
        at org.ph.javaee.training4.WorkerThread.run(WorkerThread.java:29)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)

"pool-1-thread-1" prio=6 tid=0x0000000006a3a800 nid=0x2bc runnable [0x0000000007d9e000]
   java.lang.Thread.State: RUNNABLE
        at java.util.HashMap.put(Unknown Source)
        at org.ph.javaee.training4.WorkerThread.run(WorkerThread.java:32)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)
..............

3 – CPU per thread correlation

It is now time to convert the Process Explorer thread ID DECIMAL format to HEXA format as per below. The HEXA value allows us to map and identify each thread as per below:

## TID: 1748 (nid=0X6D4)

Thread name: pool-1-thread-2
CPU @25.71%
Task: Worker thread executing a HashMap.get() operation

at java.util.HashMap.get(Unknown Source)

at org.ph.javaee.training4.WorkerThread.run(WorkerThread.java:29)

at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)

at java.lang.Thread.run(Unknown Source)

## TID: 700 (nid=0X2BC)

Thread name: pool-1-thread-1
CPU @23.55%
Task: Worker thread executing a HashMap.put() operation

at java.util.HashMap.put(Unknown Source)

at org.ph.javaee.training4.WorkerThread.run(WorkerThread.java:32)

at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)

at java.lang.Thread.run(Unknown Source)

## TID: 6304 (nid=0X18A0)

Thread name: pool-1-thread-3
CPU @12.02%
Task: Worker thread executing a HashMap.put() operation

at java.util.HashMap.put(Unknown Source)

at org.ph.javaee.training4.WorkerThread.run(WorkerThread.java:32)

at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)

at java.lang.Thread.run(Unknown Source)

## TID: 5944 (nid=0X1738)

Thread name: pool-1-thread-1
CPU @20.88%
Task: Main Java program execution

"main" prio=6 tid=0x0000000001e2b000 nid=0x1738 runnable [0x00000000029df000]

java.lang.Thread.State: RUNNABLE

at org.ph.javaee.training4.HashMapInfiniteLoopSimulator.main(HashMapInfiniteLoopSimulator.java:75)

As you can see, the above correlation and analysis is quite revealing. Our main Java program is in a hang state because our 3 worker threads are using lot of CPU and not going anywhere. They may appear "stuck" performing HashMap get() & put() but in fact they are all involved in an infinite loop condition. This is exactly what we wanted to replicate.

HashMap infinite looping deep dive

Now let’s push the analysis one step further to better understand this looping condition. For this purpose, we added tracing code within the JDK 1.7 HashMap Java class itself in order to understand what is happening. Similar logging was added for the put() operation and also a trace indicating that the internal & automatic rehash/resize got triggered.

The tracing added in get() and put() operations allows us to determine if the for() loop is dealing with circular dependency which would explain the infinite looping condition.

#### HashMap.java get() operation
public V get(Object key) {

       if (key == null)
            return getForNullKey();
        int hash = hash(key.hashCode());

/*** P-H add-on- iteration counter ***/
        int iterations = 1;
        for (Entry<K,V> e = table[indexFor(hash, table.length)];
             e != null;
             e = e.next) {

                    /*** Circular dependency check ***/
             Entry<K,V> currentEntry = e;
             Entry<K,V> nextEntry = e.next;
             Entry<K,V> nextNextEntry = e.next != null?e.next.next:null;

             K currentKey = currentEntry.key;              
             K nextNextKey = nextNextEntry != null?(nextNextEntry.key != null?nextNextEntry.key:null):null;

             System.out.println("HashMap.get() #Iterations : "+iterations++);

             if (currentKey != null && nextNextKey != null ) {       
                    if (currentKey == nextNextKey || currentKey.equals(nextNextKey))
                    System.out.println(" ** Circular Dependency detected! ["+currentEntry+"]["+nextEntry+"]"+"]["+nextNextEntry+"]");
             }
             /***** END ***/

            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
                return e.value;
        }
        return null;
    }

HashMap.get() #Iterations : 1
HashMap.put() #Iterations : 1
HashMap.put() #Iterations : 1
HashMap.put() #Iterations : 1
HashMap.put() #Iterations : 1
HashMap.resize() in progress...
HashMap.put() #Iterations : 1
HashMap.put() #Iterations : 2
HashMap.resize() in progress...
HashMap.resize() in progress...
HashMap.put() #Iterations : 1
HashMap.put() #Iterations : 2
HashMap.put() #Iterations : 1
HashMap.get() #Iterations : 1
HashMap.get() #Iterations : 1
HashMap.put() #Iterations : 1
HashMap.get() #Iterations : 1
HashMap.get() #Iterations : 1
HashMap.put() #Iterations : 1
HashMap.get() #Iterations : 1
HashMap.put() #Iterations : 1
 ** Circular Dependency detected! [362565=362565][333326=333326]][362565=362565]
HashMap.put() #Iterations : 2
 ** Circular Dependency detected! [333326=333326][362565=362565]][333326=333326]
HashMap.put() #Iterations : 1
HashMap.put() #Iterations : 1
HashMap.get() #Iterations : 1
HashMap.put() #Iterations : 1
.............................
HashMap.put() #Iterations : 56823

Again, the added logging was quite revealing. We can see that following a few internal HashMap.resize() the internal structure became affected, creating circular dependency conditions and triggering this infinite looping condition (#iterations increasing and increasing...) with no exit condition.

It is also showing that the resize() / rehash operation is the most at risk of internal corruption, especially when using the default HashMap size of 16. This means that the initial size of the HashMap appears to be a big factor in the risk & problem replication.

Finally, it is interesting to note that we were able to successfully run the test case with the non-thread safe HashMap by assigning an initial size setting at 1000000, preventing any resize at all. Find below the merged graph results:

The HashMap was our top performer but only when preventing an internal resize. Again, this is definitely not a solution to the thread safe risk but just a way to demonstrate that the resize operation is the most at risk given the entire manipulation of the HashMap performed at that time.

The ConcurrentHashMap, by far, is our overall winner by providing both fast performance and thread safety against that test case.

JBoss AS7 Map data structures usage

We will now conclude this article by looking at the different Map implementations within a modern Java EE container implementation such as JBoss AS 7.1.2. You can obtain the latest source code from the github master branch.

Find below the report:

Total JBoss AS7.1.2 Java files (August 28, 2012 snapshot): 7302
Total Java classes using java.util.Hashtable: 72
Total Java classes using java.util.HashMap: 512
Total Java classes using synchronized HashMap: 18
Total Java classes using ConcurrentHashMap: 46

Hashtable references were found mainly within the test suite components and from naming and JNDI related implementations. This low usage is not a surprise here.

References to the java.util.HashMap were found from 512 Java classes. Again not a surprise given how common this implementation is since the last several years. However, it is important to mention that a good ratio was found either from local variables (not shared across threads), synchronized HashMap or manual synchronization safeguard so “technically” thread safe and not exposed to the above infinite looping condition (pending/hidden bugs is still a reality given the complexity with Java concurrency programming…this case study involving Oracle Service Bus 11g is a perfect example).

A low usage of synchronized HashMap was found with only 18 Java classes from packages such as JMS, EJB3, RMI and clustering.

Finally, find below a breakdown of the ConcurrentHashMap usage which was our main interest here. As you will see below, this Map implementation is used by critical JBoss components layers such as the Web container, EJB3 implementation etc.

## JBoss Single Sign On

Used to manage internal SSO ID's involving concurrent Thread access

Total: 1

## JBoss Java EE & Web Container

Not surprising here since lot of internal Map data structures are used to manage the http sessions objects, deployment registry, clustering & replication, statistics etc. with heavy concurrent Thread access.

Total: 11

## JBoss JNDI & Security Layer

Used by highly concurrent structures such as internal JNDI security management.

Total: 4

## JBoss domain & managed server management, rollout plans...

Total: 7

## JBoss EJB3

Used by data structures such as File Timer persistence store, application Exception, Entity Bean cache, serialization, passivation...

Total: 8

## JBoss kernel, Thread Pools & protocol management

Used by high concurrent Threads Map data structures involved in handling and dispatching/processing incoming requests such as HTTP.

Total: 3

## JBoss connectors such as JDBC/XA DataSources...

Total: 2

## Weld (reference implementation of JSR-299: Contexts and Dependency Injection for the JavaTM EE platform)

Used in the context of ClassLoader and concurrent static Map data structures involving concurrent Threads access.

Total: 3

## JBoss Test Suite

Used in some integration testing test cases such as an internal Data Store, ClassLoader testing etc.

Total: 3

Final words

I hope this article has helped you revisit this classic problem and understand one of the common problems and risks associated with a wrong usage of the non-thread safe HashMap implementation. My main recommendation to you is to be careful when using an HashMap in a concurrent threads context. Unless you are a Java concurrency expert, I recommend that you use ConcurrentHashMap instead which offers a very good balance between performance and thread safety.

As usual, extra due diligence is always recommended such as performing cycles of load & performance testing. This will allow you to detect thread safety and / or performance problems before you promote the solution to your client production environment.

Please provide any comments and share your experience with ConcurrentHashMap or HashMap implementations and troubleshooting.

Data structure Java (programming language) Java EE Implementation Data (computing) JBoss Thread safety

Published at DZone with permission of Pierre - Hugues Charbonneau, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

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