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Why a synchronized StringBuffer was never a good idea

· Performance Zone

The Performance Zone is brought to you in partnership with AppDynamics.  See Gartner’s latest research on the application performance monitoring landscape and how APM suites are becoming more and more critical to the business.

Introduction

StringBuffer is a synchronized class for mutable strings.  The main problem with making it synchronized is that

  1. It was usually used as a local variable so making it synchronized just made it slower.
  2. It was never a good idea to use it in a multi-threaded way.  This problem is that developers assumed that methods which used StringBuffer were themselves thread safe when they were not.

The problem with StringBuffer

This is an example from a real class which is used in production in many trading systems. It's not a commonly used but you might assume that StringBuffer gives you thread safety, when it doesn't.
    private StringBuffer sb = new StringBuffer();

    public void addProperty(String name, String value) {
        if (value != null && value.length() > 0) {
            if (sb.length() > 0) {
                sb.append(',');
            }
            sb.append(name).append('=').append(value);
        }
    }
While individual calls are thread safe, multiple calls are not.  It is almost impossible to find a good use for StringBuffer that doesn't involve multiple calls (including toString)

A puzzle

Imagine three threads call (in no particular order)
 T1: addProperty("a", "b");
T2: addProperty("c", "d");
T3: sb.toString();
write a program which will generate every possible output of T3's sb.toString()  
I found 89.  With thread safety, you might reduce this to 4.
Note: if you used StringBuilder it would be worse, but at least you might not assume your method is thread safe when it is not. e.g. SimpleDateFormat uses StringBuffer ;)




The Performance Zone is brought to you in partnership with AppDynamics.  See Gartner’s latest research on the application performance monitoring landscape and how APM suites are becoming more and more critical to the business.

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Published at DZone with permission of Peter Lawrey , DZone MVB .

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