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Rework replaces previous version completely. Using ExecutorService. Use of result object instead of static variables. Ugly example is left out.

Author: thomasoss

tls/src/main/java/com/iluwatar/tls/App.java

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+/**
+ * The MIT License
+ * Copyright (c) 2016 Thomas Bauer
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.tls;
+
+import java.util.Calendar;
+import java.util.Date;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+
+/**
+ * ThreadLocal pattern
+ * <p>
+ * This App shows how to create an isolated space per each thread. In this
+ * example the usage of SimpleDateFormat is made to be thread-safe. This is an
+ * example of the ThreadLocal pattern.
+ * <p>
+ * By applying the ThreadLocal pattern you can keep track of application
+ * instances or locale settings throughout the handling of a request. The
+ * ThreadLocal class works like a static variable, with the exception that it is
+ * only bound to the current thread! This allows us to use static variables in a
+ * thread-safe way.
+ * <p>
+ * In Java, thread-local variables are implemented by the ThreadLocal class
+ * object. ThreadLocal holds a variable of type T, which is accessible via get/set
+ * methods.
+ * <p>
+ * SimpleDateFormat is one of the basic Java classes and is not thread-safe. If
+ * you do not isolate the instance of SimpleDateFormat per each thread then
+ * problems arise. 
+ * <p>
+ * App converts the String date value 15/12/2015 to the Date format using the
+ * Java class SimpleDateFormat. It does this 20 times using 4 threads, each doing 
+ * it 5 times. With the usage of as ThreadLocal in DateFormatCallable everything 
+ * runs well. But if you comment out the ThreadLocal variant (marked with "//TLTL") 
+ * and comment in the non ThreadLocal variant (marked with "//NTLNTL") you can 
+ * see what will happen without the ThreadLocal. Most likely you will get incorrect 
+ * date values and / or exceptions.
+ * <p>
+ * This example clearly show what will happen when using non thread-safe classes
+ * in a thread. In real life this may happen one in of 1.000 or 10.000 conversions
+ * and those are really hard to find errors.
+ * 
+ * @author Thomas Bauer, 2017 
+ */
+public class App {
+  /**
+   * 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) {
+      // no action here 
+    }
+    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;
+  }
+}

tls/src/main/java/com/iluwatar/tls/DateFormatCallable.java

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+/**
+ * The MIT License
+ * Copyright (c) 2016 Thomas Bauer
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.tls;
+
+import java.text.DateFormat;
+import java.text.SimpleDateFormat;
+import java.util.concurrent.Callable;
+
+/**
+ * DateFormatCallable converts string dates to a date format using
+ * SimpleDateFormat. The date format and the date value will be passed to the
+ * Callable by the constructor. The constructor creates a instance of
+ * SimpleDateFormat and stores it in a ThreadLocal class variable. For the
+ * complete description of the example see {@link App}
+ * 
+ * You can comment out the code marked with //TLTL and comment in the 
+ * code marked //NTLNTL. Then you can see what will happen if you do not
+ * use the ThreadLocal. For details see the description of {@link App}
+ *
+ * @author Thomas Bauer, 2017 
+ */
+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.Callable#call()
+   */
+  @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;
+  }
+}

tls/src/main/java/com/iluwatar/tls/Result.java

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+/*
+ * Fiducia IT AG, All rights reserved. Use is subject to license terms.
+ */
+
+package com.iluwatar.tls;
+
+import java.util.ArrayList;
+import java.util.Date;
+import java.util.List;
+
+/**
+ * Result object that will be returned by the Callable {@link DateFormatCallable} 
+ * used in {@link App}
+ *
+ * @author Thomas Bauer, 2017 
+ */
+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;
+  }
+}