java学习记录--ThreadLocal使用案例

本文借由并发环境下使用线程不安全的SimpleDateFormat优化案例,帮助大家理解ThreadLocal.

最近整理公司项目,发现不少写的比较糟糕的地方,比如下面这个:

public class DateUtil {private final static SimpleDateFormat sdfyhm = new SimpleDateFormat("yyyyMMdd");public synchronized static Date parseymdhms(String source) {try {return sdfyhm.parse(source);} catch (ParseException e) {e.printStackTrace();return new Date();}}}

首先分析下:
该处的函数parseymdhms()使用了synchronized修饰,意味着该操作是线程不安全的,所以需要同步,线程不安全也只能是SimpleDateFormat的parse()方法,查看下源码,在SimpleDateFormat里面有一个全局变量

protected Calendar calendar;Date parse() {calendar.clear();... // 执行一些操作, 设置 calendar 的日期什么的
calendar.getTime(); // 获取calendar的时间

}

该clear()操作会造成线程不安全.

此外使用synchronized 关键字对性能有很大影响,尤其是多线程的时候,每一次调用parseymdhms方法都会进行同步判断,并且同步本身开销就很大,因此这是不合理的解决方案.

改进方法

线程不安全是源于多线程使用了共享变量造成,所以这里使用ThreadLocal<SimpleDateFormat>来给每个线程单独创建副本变量,先给出代码,再分析这样的解决问题的原因.

/*** 日期工具类(使用了ThreadLocal获取SimpleDateFormat,其他方法可以直接拷贝common-lang)* @author Niu Li* @date 2016/11/19*/
public class DateUtil {private static Map<String,ThreadLocal<SimpleDateFormat>> sdfMap = new HashMap<String, ThreadLocal<SimpleDateFormat>>();private static Logger logger = LoggerFactory.getLogger(DateUtil.class);public final static String MDHMSS = "MMddHHmmssSSS";public final static String YMDHMS = "yyyyMMddHHmmss";public final static String YMDHMS_ = "yyyy-MM-dd HH:mm:ss";public final static String YMD = "yyyyMMdd";public final static String YMD_ = "yyyy-MM-dd";public final static String HMS = "HHmmss";/*** 根据map中的key得到对应线程的sdf实例* @param pattern map中的key* @return 该实例*/private static SimpleDateFormat getSdf(final String pattern){ThreadLocal<SimpleDateFormat> sdfThread = sdfMap.get(pattern);if (sdfThread == null){//双重检验,防止sdfMap被多次put进去值,和双重锁单例原因是一样的synchronized (DateUtil.class){sdfThread = sdfMap.get(pattern);if (sdfThread == null){logger.debug("put new sdf of pattern " + pattern + " to map");sdfThread = new ThreadLocal<SimpleDateFormat>(){@Overrideprotected SimpleDateFormat initialValue() {logger.debug("thread: " + Thread.currentThread() + " init pattern: " + pattern);return new SimpleDateFormat(pattern);}};sdfMap.put(pattern,sdfThread);}}}return sdfThread.get();}/*** 按照指定pattern解析日期* @param date 要解析的date* @param pattern 指定格式* @return 解析后date实例*/public static Date parseDate(String date,String pattern){if(date == null) {throw new IllegalArgumentException("The date must not be null");}try {return  getSdf(pattern).parse(date);} catch (ParseException e) {e.printStackTrace();logger.error("解析的格式不支持:"+pattern);}return null;}/*** 按照指定pattern格式化日期* @param date 要格式化的date* @param pattern 指定格式* @return 解析后格式*/public static String formatDate(Date date,String pattern){if (date == null){throw new IllegalArgumentException("The date must not be null");}else {return getSdf(pattern).format(date);}}
}

测试

在主线程中执行一个,另外两个在子线程执行,使用的都是同一个pattern

public static void main(String[] args) {DateUtil.formatDate(new Date(),MDHMSS);new Thread(()->{DateUtil.formatDate(new Date(),MDHMSS);}).start();new Thread(()->{DateUtil.formatDate(new Date(),MDHMSS);}).start();}

日志分析

put new sdf of pattern MMddHHmmssSSS to map
thread: Thread[main,5,main] init pattern: MMddHHmmssSSS
thread: Thread[Thread-0,5,main] init pattern: MMddHHmmssSSS
thread: Thread[Thread-1,5,main] init pattern: MMddHHmmssSSS

分析

可以看出来sdfMap put进去了一次,而SimpleDateFormat被new了三次,因为代码中有三个线程.那么这是为什么呢?

对于每一个线程Thread,其内部有一个ThreadLocal.ThreadLocalMap threadLocals的全局变量引用,ThreadLocal.ThreadLocalMap里面有一个保存该ThreadLocal和对应value,一图胜千言,结构图如下:

那么对于sdfMap的话,结构图就变更了下

那么日志为什么是这样的?分析下:

1.首先第一次执行`DateUtil.formatDate(new Date(),MDHMSS);`

//第一次执行DateUtil.formatDate(new Date(),MDHMSS)分析private static SimpleDateFormat getSdf(final String pattern){ThreadLocal<SimpleDateFormat> sdfThread = sdfMap.get(pattern);//得到的sdfThread为null,进入if语句if (sdfThread == null){synchronized (DateUtil.class){sdfThread = sdfMap.get(pattern);//sdfThread仍然为null,进入if语句if (sdfThread == null){//打印日志logger.debug("put new sdf of pattern " + pattern + " to map");//创建ThreadLocal实例,并覆盖initialValue方法sdfThread = new ThreadLocal<SimpleDateFormat>(){@Overrideprotected SimpleDateFormat initialValue() {logger.debug("thread: " + Thread.currentThread() + " init pattern: " + pattern);return new SimpleDateFormat(pattern);}};//设置进如sdfMap
                    sdfMap.put(pattern,sdfThread);}}}return sdfThread.get();}

这个时候可能有人会问,这里并没有调用ThreadLocal的set方法,那么值是怎么设置进入的呢?
这就需要看sdfThread.get()的实现:

public T get() {Thread t = Thread.currentThread();ThreadLocalMap map = getMap(t);if (map != null) {ThreadLocalMap.Entry e = map.getEntry(this);if (e != null) {@SuppressWarnings("unchecked")T result = (T)e.value;return result;}}return setInitialValue();}

也就是说当值不存在的时候会调用setInitialValue()方法,该方法会调用initialValue()方法,也就是我们覆盖的方法.

对应日志打印.

put new sdf of pattern MMddHHmmssSSS to map
thread: Thread[main,5,main] init pattern: MMddHHmmssSSS

2.第二次在子线程执行`DateUtil.formatDate(new Date(),MDHMSS);`

//第二次在子线程执行`DateUtil.formatDate(new Date(),MDHMSS);`private static SimpleDateFormat getSdf(final String pattern){ThreadLocal<SimpleDateFormat> sdfThread = sdfMap.get(pattern);//这里得到的sdfThread不为null,跳过if块if (sdfThread == null){synchronized (DateUtil.class){sdfThread = sdfMap.get(pattern);if (sdfThread == null){logger.debug("put new sdf of pattern " + pattern + " to map");sdfThread = new ThreadLocal<SimpleDateFormat>(){@Overrideprotected SimpleDateFormat initialValue() {logger.debug("thread: " + Thread.currentThread() + " init pattern: " + pattern);return new SimpleDateFormat(pattern);}};sdfMap.put(pattern,sdfThread);}}}//直接调用sdfThread.get()返回return sdfThread.get();}

分析sdfThread.get()

//第二次在子线程执行`DateUtil.formatDate(new Date(),MDHMSS);`public T get() {Thread t = Thread.currentThread();//得到当前子线程ThreadLocalMap map = getMap(t);//子线程中得到的map为null,跳过if块if (map != null) {ThreadLocalMap.Entry e = map.getEntry(this);if (e != null) {@SuppressWarnings("unchecked")T result = (T)e.value;return result;}}//直接执行初始化,也就是调用我们覆盖的initialValue()方法return setInitialValue();}

对应日志:

Thread[Thread-1,5,main] init pattern: MMddHHmmssSSS

总结

在什么场景下比较适合使用ThreadLocal？stackoverflow上有人给出了还不错的回答。
When and how should I use a ThreadLocal variable?
One possible (and common) use is when you have some object that is not thread-safe, but you want to avoid synchronizing access to that object (I’m looking at you, SimpleDateFormat). Instead, give each thread its own instance of the object.

参考代码:

https://github.com/nl101531/JavaWEB 下Util-Demo

参考资料:

http://www.importnew.com/21479.html
http://www.cnblogs.com/zemliu/archive/2013/08/29/3290585.html

转载于:https://www.cnblogs.com/shuilangyizu/p/8621733.html