目的
将全局变量固定到线程，以防被其他线程破坏。如果在可调用对象或可运行对象中使用非只读的类变量或静态变量，则需要这样做。
通过应用本地线程模式Thread Local Pattern，您可以在处理请求的整个过程中跟踪应用程序实例或区域设置。在Java中，ThreadLocal类的工作方式类似于静态变量，但它只绑定到当前线程！这允许我们以线程安全的方式使用静态变量。
示例
在Java中，线程局部变量由ThreadLocal类对象实现。该类提供线程局部变量。ThreadLocal包含T类型的变量，可通过get / set方法访问。例如，持有Integer值的ThreadLocal变量如下所示：
private static final ThreadLocal<Integer> myThreadLocalInteger = new ThreadLocal<Integer>();
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源代码
Java ThreadLocal类提供线程局部变量。这些变量不同于它们的正常对应变量，因为访问一个变量（通过get或set方法）的每个线程都有自己的独立初始化变量副本。ThreadLocal实例通常是希望将状态与线程（例如，用户ID或事务ID）相关联的类中的私有静态字段。通过应用ThreadLocal模式，您可以在处理请求期间跟踪应用程序实例或区域设置。ThreadLocal类的工作方式类似于静态变量，但它只绑定到当前线程！这允许我们以线程安全的方式使用静态变量。在Java中，线程局部变量由ThreadLocal类对象实现。ThreadLocal包含一个T类型的变量，可以通过get / set方法访问它。SimpleDateFormat是基本的Java类之一，不是线程安全的。如果不为每个线程隔离SimpleDateFormat实例，则会出现问题。
在本例中，simpledateformat的用法是线程安全的。这是一个Thread Local Pattern的示例。
类图
步骤1：创建DateFormatCallable类，使用SimpleDateFormat将字符串日期转换为日期格式。日期格式和日期值将由构造函数传递给Callable。构造函数创建SimpleDateFormat的实例并将其存储在ThreadLocal类变量中。
public class DateFormatCallable implements Callable<Result> {
// class variables (members)
private ThreadLocal<DateFormat> df; //TLTL
// private DateFormat df; //NTLNTL
private String dateValue; // for dateValue Thread Local not needed
/**
* The date format and the date value are passed to the constructor
*
* @param inDateFormat
* string date format string, e.g. "dd/MM/yyyy"
* @param inDateValue
* string date value, e.g. "21/06/2016"
*/
public DateFormatCallable(String inDateFormat, String inDateValue) {
final String idf = inDateFormat; //TLTL
this.df = new ThreadLocal<DateFormat>() { //TLTL
@Override //TLTL
protected DateFormat initialValue() { //TLTL
return new SimpleDateFormat(idf); //TLTL
} //TLTL
}; //TLTL
// this.df = new SimpleDateFormat(inDateFormat); //NTLNTL
this.dateValue = inDateValue;
}
/**
* @see java.util.concurrent.Callablecall()
*/
@Override
public Result call() {
System.out.println(Thread.currentThread() + " started executing...");
Result result = new Result();
// Convert date value to date 5 times
for (int i = 1; i <= 5; i++) {
try {
// this is the statement where it is important to have the
// instance of SimpleDateFormat locally
// Create the date value and store it in dateList
result.getDateList().add(this.df.get().parse(this.dateValue)); //TLTL
// result.getDateList().add(this.df.parse(this.dateValue)); //NTLNTL
} catch (Exception e) {
// write the Exception to a list and continue work
result.getExceptionList().add(e.getClass() + ": " + e.getMessage());
}
}
System.out.println(Thread.currentThread() + " finished processing part of the thread");
return result;
}
}
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步骤2：创建将由Callable DateFormatCallable返回的Result对象。
public class Result {
// A list to collect the date values created in one thread
private List<Date> dateList = new ArrayList<Date>();
// A list to collect Exceptions thrown in one threads (should be none in
// this example)
private List<String> exceptionList = new ArrayList<String>();
/**
*
* @return List of date values collected within an thread execution
*/
public List<Date> getDateList() {
return dateList;
}
/**
*
* @return List of exceptions thrown within an thread execution
*/
public List<String> getExceptionList() {
return exceptionList;
}
}
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第3步：测试。创建ThreadLocalStorageDemo类使用Java类SimpleDateFormat将String日期值15/12/2015转换为Date格式。它使用4个线程执行20次，每个线程执行5次。在DateFormatCallable中使用ThreadLocal一切都运行良好。但是如果你注释了ThreadLocal变量（标有“// TLTL”）并在非ThreadLocal变量中注释（用“// NTLNTL”标记），你可以看到没有ThreadLocal会发生什么。很可能您会得到错误的日期值及/或异常。
public class ThreadLocalStorageDemo {
/**
* Program entry point
*
* @param args
* command line args
*/
public static void main(String args) {
int counterDateValues = 0;
int counterExceptions = 0;
// Create a callable
DateFormatCallable callableDf = new DateFormatCallable("dd/MM/yyyy", "15/12/2015");
// start 4 threads, each using the same Callable instance
ExecutorService executor = Executors.newCachedThreadPool();
Future<Result> futureResult1 = executor.submit(callableDf);
Future<Result> futureResult2 = executor.submit(callableDf);
Future<Result> futureResult3 = executor.submit(callableDf);
Future<Result> futureResult4 = executor.submit(callableDf);
try {
Result result = new Result[4];
result[0] = futureResult1.get();
result[1] = futureResult2.get();
result[2] = futureResult3.get();
result[3] = futureResult4.get();
// Print results of thread executions (converted dates and raised exceptions)
// and count them
for (int i = 0; i < result.length; i++) {
counterDateValues = counterDateValues + printAndCountDates(result[i]);
counterExceptions = counterExceptions + printAndCountExceptions(result[i]);
}
// a correct run should deliver 20 times 15.12.2015
// and a correct run shouldn't deliver any exception
System.out.println("The List dateList contains " + counterDateValues + " date values");
System.out.println("The List exceptionList contains " + counterExceptions + " exceptions");
} catch (Exception e) {
System.out.println("Abnormal end of program. Program throws exception: " + e);
}
executor.shutdown();
}
/**
* Print result (date values) of a thread execution and count dates
*
* @param res contains results of a thread execution
*/
private static int printAndCountDates(Result res) {
// a correct run should deliver 5 times 15.12.2015 per each thread
int counter = 0;
for (Date dt : res.getDateList()) {
counter++;
Calendar cal = Calendar.getInstance();
cal.setTime(dt);
// Formatted output of the date value: DD.MM.YYYY
System.out.println(
cal.get(Calendar.DAY_OF_MONTH) + "." + cal.get(Calendar.MONTH) + "." + +cal.get(Calendar.YEAR));
}
return counter;
}
/**
* Print result (exceptions) of a thread execution and count exceptions
*
* @param res contains results of a thread execution
* @return number of dates
*/
private static int printAndCountExceptions(Result res) {
// a correct run shouldn't deliver any exception
int counter = 0;
for (String ex : res.getExceptionList()) {
counter++;
System.out.println(ex);
}
return counter;
}
}[/i][/i]
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适用性
在以下任何情况下使用线程本地存储
- 当您在Callable / Runnable对象中使用非只读的类变量时，在并行运行的多个线程中使用相同的Callable实例。
- 当您在Callable / Runnable对象中使用非只读的静态变量时，Callable / Runnable的多个实例可以在并行线程中运行。