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Why a synchronized StringBuffer was never a good idea

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 ;)




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