Bael 822 thread local (#1625)

* code for thread local article

* userNameSecret

* better to string

* fix typo

core-java/src/main/java/com/baeldung/threadlocal/Context.java

@@ -0,0 +1,17 @@
+package com.baeldung.threadlocal;
+
+
+public class Context {
+    private final String userName;
+
+    public Context(String userName) {
+        this.userName = userName;
+    }
+
+    @Override
+    public String toString() {
+        return "Context{" +
+                "userNameSecret='" + userName + '\'' +
+                '}';
+    }
+}

core-java/src/main/java/com/baeldung/threadlocal/SharedMapWithUserContext.java

@@ -0,0 +1,21 @@
+package com.baeldung.threadlocal;
+
+
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+public class SharedMapWithUserContext implements Runnable {
+    public final static Map<Integer, Context> userContextPerUserId = new ConcurrentHashMap<>();
+    private final Integer userId;
+    private UserRepository userRepository = new UserRepository();
+
+    public SharedMapWithUserContext(Integer userId) {
+        this.userId = userId;
+    }
+
+    @Override
+    public void run() {
+        String userName = userRepository.getUserNameForUserId(userId);
+        userContextPerUserId.put(userId, new Context(userName));
+    }
+}

core-java/src/main/java/com/baeldung/threadlocal/ThreadLocalWithUserContext.java

@@ -0,0 +1,20 @@
+package com.baeldung.threadlocal;
+
+
+public class ThreadLocalWithUserContext implements Runnable {
+    private static final ThreadLocal<Context> userContext = new ThreadLocal<>();
+    private final Integer userId;
+    private UserRepository userRepository = new UserRepository();
+
+    public ThreadLocalWithUserContext(Integer userId) {
+        this.userId = userId;
+    }
+
+
+    @Override
+    public void run() {
+        String userName = userRepository.getUserNameForUserId(userId);
+        userContext.set(new Context(userName));
+        System.out.println("thread context for given userId: " + userId + " is: " + userContext.get());
+    }
+}

core-java/src/main/java/com/baeldung/threadlocal/UserRepository.java

@@ -0,0 +1,10 @@
+package com.baeldung.threadlocal;
+
+import java.util.UUID;
+
+
+public class UserRepository {
+    public String getUserNameForUserId(Integer userId) {
+        return UUID.randomUUID().toString();
+    }
+}

core-java/src/test/java/com/baeldung/threadlocal/ThreadLocalTest.java

@@ -0,0 +1,35 @@
+package com.baeldung.threadlocal;
+
+
+import org.junit.Test;
+
+import java.util.concurrent.ExecutionException;
+
+import static org.junit.Assert.assertEquals;
+
+public class ThreadLocalTest {
+    @Test
+    public void givenThreadThatStoresContextInAMap_whenStartThread_thenShouldSetContextForBothUsers() throws ExecutionException, InterruptedException {
+        //when
+        SharedMapWithUserContext firstUser = new SharedMapWithUserContext(1);
+        SharedMapWithUserContext secondUser = new SharedMapWithUserContext(2);
+        new Thread(firstUser).start();
+        new Thread(secondUser).start();
+
+        Thread.sleep(3000);
+        //then
+        assertEquals(SharedMapWithUserContext.userContextPerUserId.size(), 2);
+    }
+
+    @Test
+    public void givenThreadThatStoresContextInThreadLocal_whenStartThread_thenShouldStoreContextInThreadLocal() throws ExecutionException, InterruptedException {
+        //when
+        ThreadLocalWithUserContext firstUser = new ThreadLocalWithUserContext(1);
+        ThreadLocalWithUserContext secondUser = new ThreadLocalWithUserContext(2);
+        new Thread(firstUser).start();
+        new Thread(secondUser).start();
+
+        Thread.sleep(3000);
+    }
+
+}