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Why a synchronized StringBuffer was never a good idea

Why a synchronized StringBuffer was never a good idea

👁 Peter Lawrey user avatar

Peter Lawrey

Apr. 25, 13 · Interview

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Introduction

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

It was usually used as a local variable so making it synchronized just made it slower.
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 ;)

dev Strings Production (computer science) IT

Published at DZone with permission of Peter Lawrey. See the original article here.

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