Learn Thread Safety and Performance Considerations | Controlling Numeric Output with DecimalFormat

When working with numeric formatting in Java, you often rely on the DecimalFormat class to control the appearance of numbers. However, it is crucial to understand that DecimalFormat is not thread-safe. This means that if you share a single instance of DecimalFormat between multiple threads, you risk encountering unpredictable behavior and incorrect formatting results. The underlying problem is that DecimalFormat maintains internal state for parsing and formatting operations. If two threads use the same instance simultaneously, their actions may interfere with each other, leading to data races and corrupted output.

The risks in a concurrent environment include:

Corrupted or incorrect formatted output;
Unexpected exceptions or errors;
Difficult-to-diagnose bugs that only appear under specific timing conditions.

To see how this can happen, examine what occurs if you share a single DecimalFormat instance across multiple threads.

Main.java


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package com.example;

import java.text.DecimalFormat;

public class Main {
    private static final DecimalFormat sharedFormat = new DecimalFormat("#,##0.00");

    public static void main(String[] args) {
        Runnable formatTask = () -> {
            for (int i = 0; i < 5; i++) {
                double value = Math.random() * 10000;
                String formatted = sharedFormat.format(value);
                System.out.println(Thread.currentThread().getName() + ": " + formatted);
            }
        };

        Thread t1 = new Thread(formatTask, "Thread-1");
        Thread t2 = new Thread(formatTask, "Thread-2");
        t1.start();
        t2.start();
    }
}

In the example above, two threads use a shared DecimalFormat instance to format random numbers. Because DecimalFormat is not thread-safe, you may see garbled output or even exceptions, especially under heavy load or on multicore machines. This demonstrates why you must take precautions when using DecimalFormat in concurrent applications.

To safely use DecimalFormat in a multi-threaded context, follow these best practices:

Use a new, local instance of DecimalFormat within each method or thread;
Synchronize access to a shared instance, but be aware that this can reduce performance;
Use a ThreadLocal<DecimalFormat> to give each thread its own instance, combining safety and efficiency.

The most common and straightforward approach is to create a new DecimalFormat object wherever you need to format numbers. This eliminates any risk of cross-thread interference and is usually fast enough for most applications. Here is how you can do this:

Main.java


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package com.example;

import java.text.DecimalFormat;

public class Main {
    public static void main(String[] args) {
        Runnable formatTask = () -> {
            DecimalFormat localFormat = new DecimalFormat("#,##0.00");
            for (int i = 0; i < 5; i++) {
                double value = Math.random() * 10000;
                String formatted = localFormat.format(value);
                System.out.println(Thread.currentThread().getName() + ": " + formatted);
            }
        };

        Thread t1 = new Thread(formatTask, "Thread-1");
        Thread t2 = new Thread(formatTask, "Thread-2");
        t1.start();
        t2.start();
    }
}

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Section 2. Chapter 4

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The risks in a concurrent environment include:

Corrupted or incorrect formatted output;
Unexpected exceptions or errors;
Difficult-to-diagnose bugs that only appear under specific timing conditions.

To see how this can happen, examine what occurs if you share a single DecimalFormat instance across multiple threads.

Main.java


              1234567891011121314151617181920212223
            
package com.example;

import java.text.DecimalFormat;

public class Main {
    private static final DecimalFormat sharedFormat = new DecimalFormat("#,##0.00");

    public static void main(String[] args) {
        Runnable formatTask = () -> {
            for (int i = 0; i < 5; i++) {
                double value = Math.random() * 10000;
                String formatted = sharedFormat.format(value);
                System.out.println(Thread.currentThread().getName() + ": " + formatted);
            }
        };

        Thread t1 = new Thread(formatTask, "Thread-1");
        Thread t2 = new Thread(formatTask, "Thread-2");
        t1.start();
        t2.start();
    }
}

To safely use DecimalFormat in a multi-threaded context, follow these best practices:

Use a new, local instance of DecimalFormat within each method or thread;
Synchronize access to a shared instance, but be aware that this can reduce performance;
Use a ThreadLocal<DecimalFormat> to give each thread its own instance, combining safety and efficiency.

Main.java


              12345678910111213141516171819202122
            
package com.example;

import java.text.DecimalFormat;

public class Main {
    public static void main(String[] args) {
        Runnable formatTask = () -> {
            DecimalFormat localFormat = new DecimalFormat("#,##0.00");
            for (int i = 0; i < 5; i++) {
                double value = Math.random() * 10000;
                String formatted = localFormat.format(value);
                System.out.println(Thread.currentThread().getName() + ": " + formatted);
            }
        };

        Thread t1 = new Thread(formatTask, "Thread-1");
        Thread t2 = new Thread(formatTask, "Thread-2");
        t1.start();
        t2.start();
    }
}

Everything was clear?

Thanks for your feedback!

Section 2. Chapter 4

Thread Safety and Performance Considerations

1. Why is `DecimalFormat` not thread-safe?

2. What is a recommended way to use `DecimalFormat` in a multi-threaded application?

3. How can ThreadLocal help with DecimalFormat usage?

Thread Safety and Performance Considerations

1. Why is `DecimalFormat` not thread-safe?

2. What is a recommended way to use `DecimalFormat` in a multi-threaded application?

3. How can ThreadLocal help with DecimalFormat usage?

Thread Safety and Performance Considerations

1. Why is DecimalFormat not thread-safe?

2. What is a recommended way to use DecimalFormat in a multi-threaded application?

3. How can ThreadLocal help with DecimalFormat usage?

Thread Safety and Performance Considerations

1. Why is DecimalFormat not thread-safe?

2. What is a recommended way to use DecimalFormat in a multi-threaded application?

3. How can ThreadLocal help with DecimalFormat usage?

1. Why is `DecimalFormat` not thread-safe?

2. What is a recommended way to use `DecimalFormat` in a multi-threaded application?

1. Why is `DecimalFormat` not thread-safe?

2. What is a recommended way to use `DecimalFormat` in a multi-threaded application?