DecimalFormat Is Not Thread Safe

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The docs are pretty clear on this point.

Decimal formats are generally not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally.

Nevertheless, because of how it’s often declared, and the evolution of its implementation, there’s an interesting gotcha here.

When declaring a DecimalFormat, this is a common pattern:

private static final DecimalFormat FORMATTER = new DecimalFormat("#,###");

It often makes sense to declare like a constant in this way, and that’s fine, as long only one thread will ever access the containing class at any given time.

However, in multi-threaded code, it can cause huge problems.

Here’s the other catch though, prior to Java 8, DecimalFormat was essentially thread safe in practice.

Java 8 introduced a fast-track option for formatting decimals with common patterns. See this issue. The performance gains are reportedly quite substantial, and as the docs had always been quite clear regarding the lack of thread safety, the change makes sense.

It did, though, bite us in some legacy code that used the above declaration pattern in multi-threaded code.

For us, it manifested as a NullPointerException, as it did for the questioner in this StackOverflow question.


In our case, the method that used the DecimalFormat was called very frequently, so I wanted to avoid instantiating a new format instance in the method body with every call.

Instead, I did exactly what the docs suggest and created separate format instances for each thread by using ThreadLocal.

The above declaration now becomes:

private static final ThreadLocal<DecimalFormat> FORMATTER =
    new ThreadLocal<DecimalFormat>() {
        protected DecimalFormat initialValue() {
            return new DecimalFormat("#,###");

And you use the formatter as so:


File Operations With Java 8 Streams

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Streams are one of the major functional improvements introduced in Java 8 (the other being lambda expressions). Reams have been written about streams in Java - the package JavaDoc is actually a pretty good starting point. In this post though, I plan to focus on a few specific use cases around file operations that I’ve found useful.

Java 7 introduced NIO.2, replacing large portions of the original file I/O capability in Java. In Java 8, NIO.2 was further extended with support for streams - offering new solutions for some use cases, two of which, I’ll be covering in this post.

  • Reading (And Acting On) Lines From a File
  • Walking a File Tree

Reading (And Acting On) Lines From a File

Reading all the lines from a file and performing some action on them has been an extremely common use case in my experience. The new lines method, on the Files class, provides a entry-point into a new method of solving this problem.

Consider this (not at all contrived) example. We are given a text file containing lines of integers like so:

1  2  3  4
5  6  7  8
1  2  4  4
4  4  1  1

And the task is to sum up all entries in the second column with a value greater than 3.

Easy enough with any version of Java, but quite verbose before streams. With streams though, one solution looks something like this:

int sum = Files.lines(path)
        .map(line -> line.split(" ")[1])
        .filter(i -> i > 3)
  • Line 1: Use the new Files.lines method to get a stream containing each line of the file.
  • Line 2: Use map to replace each line in the stream with the second entry from the line.
  • Line 3: Use mapToInt and a function reference to convert the second entry from each line into an Integer.
  • Line 4: Use filter to limit our stream to values greater than 3.
  • Line 5: Finally, call the sum method to sum up the values in the stream.

An important thing to note is that nothing is actually read from the file until sum is called. And, if something should go wrong at that point, you’ll get an UncheckedIOException.

Walking a File Tree

Another use case for Java 8 streams is when it’s necessary to walk a file tree and enumerate all the files meeting a certain criteria. The Files class again offers the entry method for this example, in the form of the walk method.

Consider this (again, not at all contrived) example. We want to recursively descend a file tree, finding all files ending with “.java”, and returning them in a list of Strings sorted alphabetically.

List<String> sortedJavaFilePaths = Files.walk(path)
        .filter(foundPath -> foundPath.toString().endsWith(".java"))
        .map(javaPath -> javaPath.getFileName().toString())
  • Line 1: Use the new Files.walk method to get a stream containing each file path located under the starting path.
  • Line 2: Use filter to limit the stream to files that end with ‘.java’
  • Line 3: Use map to convert each entry in the stream of file paths to just the file name (and converting to String along the way)
  • Line 4: Use sorted to sort the entries of the stream. In this case, the normal String sorting behavior was fine.
  • Line 5: Finally, call the collect method to collect the output of the stream into a List.
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