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ForkJoin Tasks in Java

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The fork/join framework in Java is ideal for a problem that can be divided into smaller pieces and solved in parallel. The fundamental steps of a fork/join problem are:

A ForkJoinTask is the interface that defines such a problem. It is generally expected that you will subclass one of its abstract implementations (usually the RecursiveTask) rather than implement the interface directly.

In this example, we are going to sum a collection of integers, dividing until we get to batch sizes of no more than ten. 

import java.util.List;
import java.util.concurrent.RecursiveTask;

public class SummingTask extends RecursiveTask<Integer> {
    private static final int MAX_BATCH_SIZE = 10;

    private final List<Integer> numbers;
    private final int minInclusive, maxExclusive;

    public SummingTask(List<Integer> numbers) {
        this(numbers, 0, numbers.size());
    }

    // This constructor is only used internally as part of the dividing process
    private SummingTask(List<Integer> numbers, int minInclusive, int maxExclusive) {
        this.numbers = numbers;
        this.minInclusive = minInclusive;
        this.maxExclusive = maxExclusive;
    }

    @Override
    public Integer compute() {
        if (maxExclusive - minInclusive > MAX_BATCH_SIZE) {
            // This is too big for a single batch, so we shall divide into two tasks
            int mid = (minInclusive + maxExclusive) / 2;
            SummingTask leftTask = new SummingTask(numbers, minInclusive, mid);
            SummingTask rightTask = new SummingTask(numbers, mid, maxExclusive);

            // Submit the left hand task as a new task to the same ForkJoinPool
            leftTask.fork();

            // Run the right hand task on the same thread and get the result
            int rightResult = rightTask.compute();

            // Wait for the left hand task to complete and get its result
            int leftResult = leftTask.join();

            // And combine the result
            return leftResult + rightResult;
        } else {
            // This is fine for a single batch, so we will run it here and now
            int sum = 0;
            for (int i = minInclusive; i < maxExclusive; i++) {
                sum += numbers.get(i);
            }
            return sum;
        }
    }
}

An instance of this task can now be passed to an instance of ForkJoinPool. 

// Because I am not specifying the number of threads
// it will create a thread for each available processor
ForkJoinPool pool = new ForkJoinPool();

// Submit the task to the pool, and get what is effectively the Future
ForkJoinTask<Integer> task = pool.submit(new SummingTask(numbers));

// Wait for the result
int result = task.join();
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Parallel programming with ForkJoin framework:
* Parallel programming with ForkJoin framework
* ForkJoin Tasks in Java
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0 Inheritance
1 Getting started with Java
2 Streams
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4 Collections
5 Lambda Expressions
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60 RSA Encryption
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82 Apache Commons Lang
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84 The Classpath
85 Bytecode Modification
86 XML Parsing using JAXP
87 Reference Types
88 Localization and Internationalization
89 JAX-WS
90 XML XPath Evaluation
91 Performance Tuning
92 Parallel programming with ForkJoin framework
93 Common Pitfalls
94 Non-Access Modifiers
95 Java Compiler
96 XJC
97 Installing Java Standard Edition
98 Process
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100 Dates and Time using java.time
101 Fluent Interface
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103 Just in Time JIT compiler
104 FTP File Transfer Protocol
105 Java Native Access JNI
106 Modules
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112 Pitfalss - performance
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114 Applets
115 NIO Networking
116 New File I/O
117 Secure objects
118 Pitfalls - threads and concurrency
119 Splitting a string into fixed length parts
120 Pitfalls - nulls and NullPointerException
121 SecurityManager
122 JNDI
123 super keyword
124 java.util.Objects class
125 Java command: java and javaw
126 Atomic types
127 Floating Point Operations
128 Converting to and from strings
129 sun.misc.Unsafe
130 Java Memory Model
131 Java deployment
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133 Queues and deques
134 Runtime commands
135 NumberFormat
136 Security and Cryptograhy
137 Java Virtual Machine JVM
138 Unit Testing
139 JavaBean
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142 Java SE 8 Features
143 Java SE 7 Features
144 Packages
145 Concurrency and Money
146 Concurrent Collections
147 Using ThreadPoolExecutor in multi-threaded applications
148 Java Editions, versions, releases and distributions
149 Dynamic method dispatch
150 JMX
151 Security and cryptography
152 Generating Java Code
153 JShell
154 Benchmarks
155 Collection Factory Methods
156 Multi-release JAR files
157 Stack-Walking API
158 TreeMap and TreeSet
159 Sockets
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161 Using other scripting languages in Java
162 Functional interfaces
163 List vs. set
164 2D graphics
165 Reflection
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167 Enum Map
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169 Local Inner class
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171 Immutable class
172 String Tokenizer
173 File upload to S3 / AWS
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176 Enum starting with a number
177 SortedMap
178 WeakHashMap
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180 StringBuffer
181 Choosing Collections
182 C++ Comparison
183 CompletableFuture
184 Contributors