ThreadLocal的用法

淩亂°似流年 2022-07-14 03:12 164阅读 0赞

**为什么要使用ThreadLocal**：在Java的多线程编程中，为保证多个线程对共享变量的安全访问，通常会使用synchronized来保证同一时刻只有一个线程对共享变量进行操作。这种情况下可以将类变量放到ThreadLocal类型的对象中，使变量在每个线程中都有独立拷贝，不会出现一个线程读取变量时而被另一个线程修改的现象。

简单的来说，ThreadLocal对于不同线程中的变量不共享，每个线程都有一个自己的。

public class ConnThreadLocal {
    
        public static ThreadLocal<String> th = new ThreadLocal<String>();
    
        public void setTh(String value){
            th.set(value);
        }
        public void getTh(){
            System.out.println(Thread.currentThread().getName() + ":" + this.th.get());
        }
    
        public static void main(String[] args) throws InterruptedException {
    
            final ConnThreadLocal ct = new ConnThreadLocal();
            Thread t1 = new Thread(new Runnable() {
                @Override
                public void run() {
                    ct.setTh("张三");
                    ct.getTh();
                }
            }, "t1");
    
            Thread t2 = new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(1000);
                        ct.setTh("李四");
                        ct.getTh();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }, "t2");
    
            t1.start();
            t2.start();
        }
    
    }

输出的两个的到的值是不一样的，说明他们都有自己的ThreadLocal，他们并没有共享一个。

然后附上一个一个使用ThreadLocal的实例，数据库连接池（待修饰版，简版）

import java.sql.DriverManager;
    import java.sql.SQLException;
    import java.util.ArrayList;
    import java.util.HashMap;
    import java.util.List;
    
    import javassist.expr.NewArray;
    import com.mysql.jdbc.Connection;
    import scala.annotation.meta.getter;
    import scala.math.Numeric.IntIsIntegral;
    
    public class MyConnectionPool { 
        private int minConn = 10; //初始化连接池的最小连接数
        private int maxConn = 1000; //连接池的最大连接数量
        private String url;
        private String userName;
        private String password;
        private static MyConnectionPool myConnectionPool;
        private HashMap<String,Connection> connMap = new HashMap<String,Connection>();
        private List<String> connList = new ArrayList<String>();
        private List<String> busyList = new ArrayList<String>();
        /** * 私有化构造方法，进行单例创建连接池 */
        private MyConnectionPool(int minConn, int maxConn) throws Exception {
            this.minConn = minConn;
            this.maxConn = maxConn;
            init();
        }
        /** * 初始化连接池 */
        private void init() throws Exception {
            for (int i = 0; i < minConn; i++) {
                createConnection();
            }
        }
        /** * 的到连接池对象的方法 */
        public static MyConnectionPool getInstance(int minConn, int maxConn) throws Exception {
            if (myConnectionPool == null) {
                myConnectionPool = new MyConnectionPool(minConn, maxConn);
            }
            return myConnectionPool;
        }
        /** * 获得连接池 * @return 返回一个连接池的实例 */
        public Connection getConnetion() throws ClassNotFoundException,
                SQLException, InterruptedException {
            if (connList.size() == 0 && busyList.size() >= maxConn) {
                while (true) {
                    Thread.sleep(1000);
    
                    if (connList.size() > 0) {
                        break;
                    }
                }
    
            }
            if (connList.size() == 0 && busyList.size() < maxConn) {
                createConnection();
            }
            busyList.add(connList.get(0));
            Connection conn = connMap.get(connList.get(0));
    
            ThreadLocal<Connection> tl = new ThreadLocal<>();
            ThreadLocal<String> t2 = new ThreadLocal<>();
            tl.set(conn);
            t2.set(connList.get(0));
            connList.remove(0);
            return tl.get();
    
        }
        /** * 连接池回收 */
        public void collectConnect() {
            ThreadLocal<String> tl = new ThreadLocal<>();
            String id=tl.get();
            busyList.remove(id);
            connList.add(id);
    
        }
        /** * 创建单个连接 */
        public void createConnection() throws ClassNotFoundException, SQLException {
            Class.forName("com.mysql.jdbc.Driver");
            Connection conn = (Connection) DriverManager.getConnection(url,
                    userName, password);
            String id=System.currentTimeMillis()+"";
            connList.add(id);
            connMap.put(id, conn);
        }
    
    }