ThreadLocal使用代码欣赏

清疚 2022-09-27 12:29 172阅读 0赞

使用ThreadLocal可以用来和线程绑定特定的变量，不能被其它的线程使用。来看一个例子：

public class DateUtils { 
    
        public static final String DATE_FMT = "yyyy-MM-dd";
    
        public static final String DATE_FMT1 = "yyyy-MM-dd HH:mm:ss";
    
        private static Map<String, ThreadLocal<SimpleDateFormat>> formatMap = Maps.newConcurrentMap();
    
        public static Date toDate(String date) {
            return toDate(date, DATE_FMT);
        }
    
        public static Date toDate(String date, String fmt) {
            if (StringUtils.isBlank(date)) {
                return null;
            }
            try {
                final SimpleDateFormat sdf = getFormat(fmt);
                return sdf.parse(date);
            } catch (Exception ex) {
                return null;
            }
        }
    
        public static String format(Date date) {
            return format(date, DATE_FMT);
        }
    
        public static String format(Date date, String fmt) {
            if (date == null) {
                return null;
            }
            try {
                final SimpleDateFormat sdf = getFormat(fmt);
                return sdf.format(date);
            } catch (Exception ex) {
                return null;
            }
        }
    
        public static SimpleDateFormat getFmt(String fmt) {
            return getFormat(fmt);
        }
    
        private static SimpleDateFormat getFormat(final String fmt) {
            ThreadLocal<SimpleDateFormat> fmtLocal = formatMap.get(fmt);
            if (fmtLocal == null) {
                synchronized (DateUtils.class) {
                    fmtLocal = formatMap.get(fmt);
                    if (fmtLocal == null) { // Double-Check
                        fmtLocal = new ThreadLocal<SimpleDateFormat>() {
                            @Override
                            protected SimpleDateFormat initialValue() {
                                return new SimpleDateFormat(fmt);
                            };
                        };
                        formatMap.put(fmt, fmtLocal);
                    }
                }
            }
            return fmtLocal.get();
        }
    }

**关注其中的getFormat方法**  
1）SimpleDateFormat不是线程安全的；  
2）定义Map key是dateFormat，value是ThreadLocal对象；  
3）使用Double-check的方法为每个线程创建SimpleDateFormat单例；  
4）内部类重写ThreadLocal initialValue方法，返回new SimpleDateFormat；  
5）调用ThreadLocal get方法获取SimpleDateFormat，具体看ThreadLocal中get方法的实现，及其调用关系；

public class ThreadLocal<T> { 
        /** * Returns the current thread's "initial value" for this * thread-local variable. This method will be invoked the first * time a thread accesses the variable with the {@link #get} * method, unless the thread previously invoked the {@link #set} * method, in which case the <tt>initialValue</tt> method will not * be invoked for the thread. Normally, this method is invoked at * most once per thread, but it may be invoked again in case of * subsequent invocations of {@link #remove} followed by {@link #get}. * * <p>This implementation simply returns <tt>null</tt>; if the * programmer desires thread-local variables to have an initial * value other than <tt>null</tt>, <tt>ThreadLocal</tt> must be * subclassed, and this method overridden. Typically, an * anonymous inner class will be used. * * @return the initial value for this thread-local */
        protected T initialValue() {
            return null;
        }
        /** * Returns the value in the current thread's copy of this * thread-local variable. If the variable has no value for the * current thread, it is first initialized to the value returned * by an invocation of the {@link #initialValue} method. * * @return the current thread's value of this thread-local */
        public T get() {
            Thread t = Thread.currentThread();
            ThreadLocalMap map = getMap(t);
            if (map != null) {
                ThreadLocalMap.Entry e = map.getEntry(this);
                if (e != null)
                    return (T)e.value;
            }
            return setInitialValue();
        }
    
        /** * Get the map associated with a ThreadLocal. Overridden in * InheritableThreadLocal. * * @param t the current thread * @return the map */
        ThreadLocalMap getMap(Thread t) {
            return t.threadLocals;
        }
        /** * Variant of set() to establish initialValue. Used instead * of set() in case user has overridden the set() method. * * @return the initial value */
        private T setInitialValue() {
            T value = initialValue();
            Thread t = Thread.currentThread();
            ThreadLocalMap map = getMap(t);
            if (map != null)
                map.set(this, value);
            else
                createMap(t, value);
            return value;
        }
        /** * Create the map associated with a ThreadLocal. Overridden in * InheritableThreadLocal. * * @param t the current thread * @param firstValue value for the initial entry of the map * @param map the map to store. */
        void createMap(Thread t, T firstValue) {
            t.threadLocals = new ThreadLocalMap(this, firstValue);
        }
    }

6）threadLocals为 Thread类中的属性，由ThreadLocal类来维护。

public
    class Thread implements Runnable { 
        /* ThreadLocal values pertaining to this thread. This map is maintained * by the ThreadLocal class. */
        ThreadLocal.ThreadLocalMap threadLocals = null;
    }