提问一道scjp的题。

[「已注销」]

Question 90
Given:
1. public class MyLogger {
2. private StringBuilder logger = new StringBuuilder();
3. public void log(String message, String user) {
4. logger.append(message);
5. logger.append(user);
6. }
7. }
The programmer must guarantee that a single MyLogger object works
properly for a multi-threaded system. How must this code be changed
to be thread-safe?
A. synchronize the log method
B. replace StringBuilder with StringBuffer
C. No change is necessary, the current MyLogger code is already
thread-safe.
D. replace StringBuilder with just a String object and use the string
concatenation (+=) within the log method
Answer: A大家帮帮忙解释一下

[wxzyydy]

帮楼主顶顶，答案是选A

[blliy117]

A. synchronize the log method
B. replace StringBuilder with StringBuffer
C. No change is necessary, the current MyLogger code is already
thread-safe.
D. replace StringBuilder with just a String object and use the string
concatenation (+=) within the log method
Answer: A大家帮帮忙解释一下

A. synchronize the log method 意思就是public void log(String message, String user)
--》public synchronize void log(String message, String user)
我感觉也是A

[苍蝇①号]

因为加了synchronize后，就等于在对象上加了锁，锁的粒度是整个对象。所以在多线程环境中只可能一个线程获得此对象锁，也就说只能一个接一个的线程来访问此对象，因此答案为A 别的三个都非线程安全

[「已注销」]

[Quote=引用 2 楼 blliy117 的回复:]
A. synchronize the log method
B. replace StringBuilder with StringBuffer
C. No change is necessary, the current MyLogger code is already
thread-safe.
D. replace StringBuilder with just a String object and use the string
concatenation (+=) within the log method
Answer: A大家帮帮忙解释一下

A. synchronize the log method 意思就是public void log(String message, String user)
--》public synchronize  void log(String message, String user)
我感觉也是A
[/Quote]那B为什么不行呢

[zxj828282]

那B为什么不行呢

[Sodino]

虽然StringBuffer是thread-safe的，但是用于本题却不代表执行结果是thread-safe.

从本题的意思来看，原子操作为：
4. logger.append(message);
5. logger.append(user);
4，5步依次一次性执行或要嘛都不执行。

在multi-thread,如果多个线程都申请调用public void log(String message, String user)方法，在该方法没有加synchronized标识符的情况下，可能会出现message和user不是成对依次加入的情况。可能会出现
mes mes user user这样的的错误的结构。

所以选择A,对logger的操作是线程安全的。选B无法保证。