ThreadLocal有哪些使用场景？
7.1 多线程场景下共享变量问题
ThreadLocal是线程本地变量，可以存储共享变量副本，每一个独立线程都有与共享变量一模一样的副本。ThreadLocal在当前线程下共享变量是全局共享的，各个线程之间是相互独立的。
ThreadLocal在多线程场景下解决共享变量问题代码案例：
public class SharedVariableExample {private static ThreadLocal<Integer> sharedVariable = new ThreadLocal<>();public static void main(String[] args) {ExecutorService executorService = Executors.newFixedThreadPool(3);for (int i = 0; i < 3; i++) {final int value = i; // 保存当前值，确保每个线程的值不同executorService.submit(() -> {sharedVariable.set(value); // 将值设置到ThreadLocal中try {processValue(); // 处理共享变量} finally {sharedVariable.remove(); // 在任务完成后清除ThreadLocal的值}});}executorService.shutdown();}private static void processValue() {int value = sharedVariable.get(); // 从ThreadLocal中获取值System.out.println("Thread " + Thread.currentThread().getName() + ": Value = " + value);}
}
执行结果如下，每个线程都有自己独立的共享变量副本，并且在当前线程下任务一个地方值都是一样的(一个线程下，可能存在多个方法，多个方法即当前线程下共享变量全局共享)
7.2 保存系统上下文信息
在多线程环境中，有时需要在线程之间传递数据，但不希望通过方法参数或全局变量来传递。ThreadLocal可以在当前线程中存储数据，其他线程可以通过ThreadLocal获取该数据。
使用ThreadLocal实现保存上下文信息代码案例如下
新建User类
public class User {private String username;public User(String username) {this.username = username;}public String getUsername() {return username;}
}
新建RequestContext类，用于保存信息到ThreadLocal中
public class RequestContext {private static ThreadLocal<RequestContext> contextHolder = new ThreadLocal<>();private String requestId;private User currentUser;private RequestContext(String requestId, User currentUser) {this.requestId = requestId;this.currentUser = currentUser;}public static void setCurrentContext(String requestId, User currentUser) {RequestContext context = new RequestContext(requestId, currentUser);contextHolder.set(context);}public static RequestContext getCurrentContext() {return contextHolder.get();}public String getRequestId() {return requestId;}public User getCurrentUser() {return currentUser;}
}
新建UserService处理请求
public class UserService {public void processRequest() {RequestContext context = RequestContext.getCurrentContext();String requestId = context.getRequestId();User currentUser = context.getCurrentUser();System.out.println("Processing request: " + requestId + " ,Current user: " + currentUser.getUsername());}
}
模拟两个请求，分别由不同的两个用户发起的请求。UserService类和RequestContext类本身没有直接的关系，从程序运行结果来看，信息确实从RequestContext透传到了UserService中，说明ThreadLocal起到了中间作用，可以用来保存系统上下文信息。
public class Main {public static void main(String[] args) {// 模拟请求1User user1 = new User("Alice");RequestContext.setCurrentContext("request-1", user1);UserService userService = new UserService();userService.processRequest();// 模拟请求2User user2 = new User("Bob");RequestContext.setCurrentContext("request-2", user2);userService.processRequest();}
}
7.3 管理数据库连接
public class ConnectionManager {private static ThreadLocal<Connection> connectionHolder = new ThreadLocal<>();public static Connection getConnection() throws SQLException {// 从ThreadLocal获取连接Connection connection = connectionHolder.get();// 如果连接不存在，则创建新连接并保存到ThreadLocalif (connection == null || connection.isClosed()) {connection = createConnection();connectionHolder.set(connection);}return connection;}private static Connection createConnection() throws SQLException {// 创建数据库连接String url = "jdbc:mysql://localhost:3306/dev";String username = "root";String password = "root";return DriverManager.getConnection(url, username, password);}public static void closeConnection() throws SQLException {// 关闭连接并从ThreadLocal中移除Connection connection = connectionHolder.get();if (connection != null && !connection.isClosed()) {connection.close();}connectionHolder.remove();}
}
public class DatabaseService {public void performDatabaseOperation() throws SQLException {Connection connection = ConnectionManager.getConnection();// 使用连接执行数据库操作// ...// 操作完成后关闭连接ConnectionManager.closeConnection();}
}
public class Main {public static void main(String[] args) throws SQLException {// 创建多个线程模拟并发访问Thread thread1 = new Thread(() -> {try {DatabaseService service = new DatabaseService();service.performDatabaseOperation();} catch (SQLException e) {e.printStackTrace();}});Thread thread2 = new Thread(() -> {try {DatabaseService service = new DatabaseService();service.performDatabaseOperation();} catch (SQLException e) {e.printStackTrace();}});// 启动线程thread1.start();thread2.start();}
}
一个请求对应一个数据库连接，一个请求下的所有对数据库的操作都是基于该连接进行的。这样可以在一定程度上避免频繁的创建和销毁数据库连接，从而提高性能。
7.4 基于ThreadLocal实现事务功能
使用ThreadLocal实现事务注解的原理是通过在每个线程中维护一个事务上下文对象，将事务状态与当前线程绑定起来。当需要开启事务时，通过注解或编程方式将事务上下文对象与当前线程进行关联，以便在整个事务执行过程中使用相同的事务上下文对象。
首先，定义一个事务上下文对象，用于存储事务相关的信息，例如事务状态、连接对象等。
public class TransactionContext {private Connection connection;private boolean inTransaction;// 省略构造方法和其他属性的访问方法public Connection getConnection() {return connection;}public void setConnection(Connection connection) {this.connection = connection;}public boolean isInTransaction() {return inTransaction;}public void setInTransaction(boolean inTransaction) {this.inTransaction = inTransaction;}
}
接下来，定义一个事务管理器类，使用ThreadLocal来存储和获取当前线程的事务上下文对象。
package com.spring6.learn.ThreadLocal.test6;import java.sql.Connection;
import java.sql.SQLException;public class TransactionManager {private static ThreadLocal<TransactionContext> transactionContextHolder = new ThreadLocal<>();public static void beginTransaction() {TransactionContext context = new TransactionContext();transactionContextHolder.set(context);context.setInTransaction(true);}public static void commitTransaction() {TransactionContext context = transactionContextHolder.get();if (context != null && context.isInTransaction()) {Connection connection = context.getConnection();try {connection.commit();} catch (SQLException e) {e.printStackTrace();}context.setInTransaction(false);closeConnection(connection);}}public static void rollbackTransaction() {TransactionContext context = transactionContextHolder.get();if (context != null && context.isInTransaction()) {Connection connection = context.getConnection();try {connection.rollback();} catch (SQLException e) {e.printStackTrace();}context.setInTransaction(false);closeConnection(connection);}}public static Connection getCurrentConnection() {TransactionContext context = transactionContextHolder.get();if (context != null) {return context.getConnection();}return null;}public static void setCurrentConnection(Connection connection) {TransactionContext context = transactionContextHolder.get();if (context == null) {context = new TransactionContext();transactionContextHolder.set(context);}context.setConnection(connection);}private static void closeConnection(Connection connection) {try {if (connection != null && !connection.isClosed()) {connection.close();}} catch (SQLException e) {e.printStackTrace();}}
}
public class TransactionManagerTest {private Connection connection;@Beforepublic void setUp() throws SQLException {connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/mysql", "root", "root");TransactionManager.setCurrentConnection(connection);TransactionManager.beginTransaction();}@Afterpublic void tearDown() throws SQLException {TransactionManager.rollbackTransaction();TransactionManager.setCurrentConnection(null);if (connection != null && !connection.isClosed()) {connection.close();}}@Testpublic void testTransaction() throws SQLException {// 在事务中插入一条数据String insertQuery = "INSERT INTO mytable (name) VALUES (?)";try (PreparedStatement statement = connection.prepareStatement(insertQuery)) {statement.setString(1, "John Doe");statement.executeUpdate();}// 在事务中查询数据String selectQuery = "SELECT COUNT(*) FROM mytable";try (PreparedStatement statement = connection.prepareStatement(selectQuery)) {ResultSet resultSet = statement.executeQuery();if (resultSet.next()) {int count = resultSet.getInt(1);assertEquals(1, count); // 验证插入的数据是否存在}}}
}
TransactionManager.beginTransaction();
}
@After
public void tearDown() throws SQLException {TransactionManager.rollbackTransaction();TransactionManager.setCurrentConnection(null);if (connection != null && !connection.isClosed()) {connection.close();}
}@Test
public void testTransaction() throws SQLException {// 在事务中插入一条数据String insertQuery = "INSERT INTO mytable (name) VALUES (?)";try (PreparedStatement statement = connection.prepareStatement(insertQuery)) {statement.setString(1, "John Doe");statement.executeUpdate();}// 在事务中查询数据String selectQuery = "SELECT COUNT(*) FROM mytable";try (PreparedStatement statement = connection.prepareStatement(selectQuery)) {ResultSet resultSet = statement.executeQuery();if (resultSet.next()) {int count = resultSet.getInt(1);assertEquals(1, count); // 验证插入的数据是否存在}}
}
}
