本文借由并發環境下使用線程不安全的SimpleDateFormat優化案例,幫助大家理解ThreadLocal.

最近整理公司項目,發現不少寫的比較糟糕的地方,比如下面這個:

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:
該處的函數parseymdhms()使用了synchronized修飾,意味著該操作是線程不安全的,所以需要同步,線程不安全也只能是SimpleDateFormat的parse()方法,查看下源碼,在SimpleDateFormat里面有一個全局變量

protected Calendar calendar;Date parse() {calendar.clear();... // 執行一些操作, 設置 calendar 的日期什么的 calendar.getTime(); // 獲取calendar的時間 }

該clear()操作會造成線程不安全.

此外使用synchronized 關鍵字對性能有很大影響,尤其是多線程的時候,每一次調用parseymdhms方法都會進行同步判斷,并且同步本身開銷就很大,因此這是不合理的解決方案.

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改進方法

線程不安全是源于多線程使用了共享變量造成,所以這里使用ThreadLocal<SimpleDateFormat>來給每個線程單獨創建副本變量,先給出代碼,再分析這樣的解決問題的原因.

/*** 日期工具類(使用了ThreadLocal獲取SimpleDateFormat,其他方法可以直接拷貝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";/*** 根據map中的key得到對應線程的sdf實例* @param pattern map中的key* @return 該實例*/private static SimpleDateFormat getSdf(final String pattern){ThreadLocal<SimpleDateFormat> sdfThread = sdfMap.get(pattern);if (sdfThread == null){//雙重檢驗,防止sdfMap被多次put進去值,和雙重鎖單例原因是一樣的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實例*/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);}} }

測試

在主線程中執行一個,另外兩個在子線程執行,使用的都是同一個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

可以看出來sdfMap put進去了一次,而SimpleDateFormat被new了三次,因為代碼中有三個線程.那么這是為什么呢?

對于每一個線程Thread,其內部有一個ThreadLocal.ThreadLocalMap threadLocals的全局變量引用,ThreadLocal.ThreadLocalMap里面有一個保存該ThreadLocal和對應value,一圖勝千言,結構圖如下:

那么對于sdfMap的話,結構圖就變更了下

那么日志為什么是這樣的?分析下:

1.首先第一次執行DateUtil.formatDate(new Date(),MDHMSS);

//第一次執行DateUtil.formatDate(new Date(),MDHMSS)分析private static SimpleDateFormat getSdf(final String pattern){ThreadLocal<SimpleDateFormat> sdfThread = sdfMap.get(pattern);//得到的sdfThread為null,進入if語句if (sdfThread == null){synchronized (DateUtil.class){sdfThread = sdfMap.get(pattern);//sdfThread仍然為null,進入if語句if (sdfThread == null){//打印日志logger.debug("put new sdf of pattern " + pattern + " to map");//創建ThreadLocal實例,并覆蓋initialValue方法sdfThread = new ThreadLocal<SimpleDateFormat>(){@Overrideprotected SimpleDateFormat initialValue() {logger.debug("thread: " + Thread.currentThread() + " init pattern: " + pattern);return new SimpleDateFormat(pattern);}};//設置進如sdfMap sdfMap.put(pattern,sdfThread);}}}return sdfThread.get();}

這個時候可能有人會問,這里并沒有調用ThreadLocal的set方法,那么值是怎么設置進入的呢?
這就需要看sdfThread.get()的實現:

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();}

也就是說當值不存在的時候會調用setInitialValue()方法,該方法會調用initialValue()方法,也就是我們覆蓋的方法.

對應日志打印.

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

2.第二次在子線程執行DateUtil.formatDate(new Date(),MDHMSS);

//第二次在子線程執行`DateUtil.formatDate(new Date(),MDHMSS);`private static SimpleDateFormat getSdf(final String pattern){ThreadLocal<SimpleDateFormat> sdfThread = sdfMap.get(pattern);//這里得到的sdfThread不為null,跳過if塊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);}}}//直接調用sdfThread.get()返回return sdfThread.get();}

分析sdfThread.get()

//第二次在子線程執行`DateUtil.formatDate(new Date(),MDHMSS);`public T get() {Thread t = Thread.currentThread();//得到當前子線程ThreadLocalMap map = getMap(t);//子線程中得到的map為null,跳過if塊if (map != null) {ThreadLocalMap.Entry e = map.getEntry(this);if (e != null) {@SuppressWarnings("unchecked")T result = (T)e.value;return result;}}//直接執行初始化,也就是調用我們覆蓋的initialValue()方法return setInitialValue();}

對應日志:

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

總結

在什么場景下比較適合使用ThreadLocal？stackoverflow上有人給出了還不錯的回答。 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.

參考代碼:

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

?

參考資料:

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

轉載于:https://www.cnblogs.com/shuilangyizu/p/8621733.html

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