Question

My application is multithreaded with intensive String processing. We are experiencing excessive memory consumption and profiling has demonstrated that this is due to String data. I think that memory consumption would benefit greatly from using some kind of flyweight pattern implementation or even cache (I know for sure that Strings are often duplicated, although I don’t have any hard data in that regard).

I have looked at Java Constant Pool and String.intern, but it seems that it can provoke some PermGen problems.

What would be the best alternative for implementing application-wide, multithreaded pool of Strings in java?

EDIT: Also see my previous, related question: How does java implement flyweight pattern for string under the hood?

Answer 1

Note: This answer uses examples that might not be relevant in modern runtime JVM libraries. In particular, the substring example is no longer an issue in OpenJDK/Oracle 7+.

I know it goes against what people often tell you, but sometimes explicitly creating new String instances can be a significant way to reduce your memory.

Because Strings are immutable, several methods leverage that fact and share the backing character array to save memory. However, occasionally this can actually increase the memory by preventing garbage collection of unused parts of those arrays.

For example, assume you were parsing the message IDs of a log file to extract warning IDs. Your code would look something like this:

//Format:
//ID: [WARNING|ERROR|DEBUG] Message...
String testLine = "5AB729: WARNING Some really really really long message";

Matcher matcher = Pattern.compile("([A-Z0-9]*): WARNING.*").matcher(testLine);
if ( matcher.matches() ) {
    String id = matcher.group(1);
        //...do something with id...
}

But look at the data actually being stored:

    //...
    String id = matcher.group(1);
    Field valueField = String.class.getDeclaredField("value");
    valueField.setAccessible(true);

    char[] data = ((char[])valueField.get(id));
    System.out.println("Actual data stored for string \"" + id + "\": " + Arrays.toString(data) );

It’s the whole test line, because the matcher just wraps a new String instance around the same character data. Compare the results when you replace String id = matcher.group(1); with String id = new String(matcher.group(1));.