Thread Local Storage issue #77

pom.xml

@@ -21,7 +21,7 @@
     <modelVersion>4.0.0</modelVersion>
     <groupId>com.iluwatar</groupId>
     <artifactId>java-design-patterns</artifactId>
-    <version>1.14.0-SNAPSHOT</version>
+    <version>1.15.0-SNAPSHOT</version>
     <packaging>pom</packaging>
     <inceptionYear>2014</inceptionYear>
     <properties>
@@ -50,6 +50,7 @@
     </properties>
     <modules>
         <module>abstract-factory</module>
+        <module>tls</module>
         <module>builder</module>
         <module>factory-method</module>
         <module>prototype</module>
@@ -466,4 +467,4 @@
         </plugins>
     </reporting>
 
-</project>
+</project>

tls/README.md

@@ -0,0 +1,22 @@
+---
+layout: pattern
+title: Thread Local Storage
+folder: tls
+permalink: /patterns/tls/
+pumlid: 
+categories: Concurrency
+tags:
+ - Java
+ - Difficulty-Intermediate
+---
+
+## Intent
+Securing variables global to a thread against being spoiled by other threads. That is needed if you use class variables or static variables in your Callable object or Runnable object that are not read-only.
+
+![alt text](./etc/tls.png "Thread Local Storage")
+
+## Applicability
+Use the Thread Local Storage in any of the following situations
+
+* when you use class variables in your Callable / Runnalbe object that are not read-only and you use the same Callable instance in more than one thread running in parallel
+* when you use static variables in your Callable / Runnable object that are not read-only and more than one instances of the Callable / Runnalbe may run in parallel threads.

tls/etc/tls.ucls

@@ -0,0 +1,79 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<class-diagram version="1.1.13" icons="true" always-add-relationships="false" generalizations="true" realizations="true" 
+  associations="true" dependencies="false" nesting-relationships="true" router="FAN">  
+  <class id="1" language="java" name="com.iluwatar.tls.DateFormatCallable" project="PatternIluwatar" 
+    file="/PatternIluwatar/src/main/java/com/iluwatar/tls/DateFormatCallable.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="503" y="227"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <interface id="2" language="java" name="java.util.concurrent.Callable" project="Intros" 
+    file="C:/interne/Programme/jdk8u60x64/jre/lib/rt.jar" binary="true" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="501" y="52"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </interface>  
+  <class id="3" language="java" name="java.lang.ThreadLocal" project="Intros" 
+    file="C:/interne/Programme/jdk8u60x64/jre/lib/rt.jar" binary="true" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="111" y="284"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="4" language="java" name="com.iluwatar.tls.App" project="PatternIluwatar" 
+    file="/PatternIluwatar/src/main/java/com/iluwatar/tls/App.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="845" y="228"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="5" language="java" name="com.iluwatar.tls.Result" project="PatternIluwatar" 
+    file="/PatternIluwatar/src/main/java/com/iluwatar/tls/Result.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="673" y="459"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <dependency id="6">    
+    <end type="SOURCE" refId="1"/>    
+    <end type="TARGET" refId="5"/>  
+  </dependency>  
+  <association id="7">    
+    <end type="SOURCE" refId="1" navigable="false">      
+      <attribute id="8" name="df"/>      
+      <multiplicity id="9" minimum="0" maximum="1"/>    
+    </end>    
+    <end type="TARGET" refId="3" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <realization id="10">    
+    <end type="SOURCE" refId="1"/>    
+    <end type="TARGET" refId="2"/>  
+  </realization>  
+  <dependency id="11">    
+    <end type="SOURCE" refId="4"/>    
+    <end type="TARGET" refId="5"/>  
+  </dependency>  
+  <dependency id="12">    
+    <end type="SOURCE" refId="4"/>    
+    <end type="TARGET" refId="1"/>  
+  </dependency>  
+  <classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+    sort-features="false" accessors="true" visibility="true">    
+    <attributes public="true" package="true" protected="true" private="true" static="true"/>    
+    <operations public="true" package="true" protected="true" private="true" static="true"/>  
+  </classifier-display>  
+  <association-display labels="true" multiplicity="true"/>
+</class-diagram>

tls/etc/tls.urm.puml

@@ -0,0 +1,23 @@
+@startuml
+package com.iluwatar.tls {
+  class App {
+    + App()
+    + main(args : String[]) {static}
+    - printAndCountDates(res : Result) : int {static}
+    - printAndCountExceptions(res : Result) : int {static}
+  }
+  class DateFormatCallable {
+    - dateValue : String
+    - df : ThreadLocal<DateFormat>
+    + DateFormatCallable(inDateFormat : String, inDateValue : String)
+    + call() : Result
+  }
+  class Result {
+    - dateList : List<Date>
+    - exceptionList : List<String>
+    + Result()
+    + getDateList() : List<Date>
+    + getExceptionList() : List<String>
+  }
+}
+@enduml

tls/pom.xml

@@ -0,0 +1,43 @@
+<?xml version="1.0"?>
+<!--
+
+    The MIT License
+    Copyright (c) 2014-2016 Ilkka Seppälä
+    2016 adapted for tls 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.
+
+-->
+<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.15.0-SNAPSHOT</version>
+  </parent>
+  <artifactId>tls</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>

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

@@ -0,0 +1,146 @@
+/**
+ * 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) {
+      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;
+  }
+}