ThreadLocal

优势：

1. 传递数据：在需要的地方获取数据，避免参数传递带来的代码耦合问题
2. 线程隔离：在各线程的数据相互隔离又能并发，避免了同步方式带来的性能损失

基本使用

public class Test {
    ThreadLocal<String> t1 = new ThreadLocal<>();
    public String getContent() {
        return t1.get();
    }
    public void setContent(String content) {
        t1.set(content);
    }
}

public class AccountService {
    public boolean transfer(String out, String in, int money) {
        AccountDao dao = new AccountDao();
        Connection conn = null;
        try {
            synchronized (AccountService.class) {
                conn = JdbcUtil.getConnection();
                // 关闭自动提交，开启事务
                conn.setAutoCommit(false);
                /* 事务使用注意点： 1. 保证两次数据库操作的conn对象一致 2. 线程隔离 常规解决方案： 1. 传参：service层的conn传到dao层 2. 加锁（别的线程不能在此时getConn，否则可能导致当前线程的conn发生改变） 常规解决方案的弊端： 1. 提高代码耦合度 2. 降低程序性能 */
                dao.out(out, money, conn);
                dao.in(in, money, conn);
                conn.commit();
                conn.close();
                return true;
            }
        } catch (Exception e) {
            conn.rollback();
            conn.close();
            return false;
        }
    }
}
/** * 1. 方法添加参数 conn，不能从连接池获取conn * 2. dao层不能释放连接 */
public class AccountDao {
    // 转出
    public void out(String out, int money, Connection conn) {
        String sql = "...";
        PreparedStatement pstm = conn.prepareStatement(sql);
        // ...
        pstm.executeUpdate();
    }
    // 转入
    public void in(String in, int money, Connection conn) {
        String sql = "...";
        PreparedStatement pstm = conn.prepareStatement(sql);
        // ...
        pstm.executeUpdate();
    }
}

改进

public class JdbcUtils {
    ThreadLocal<Connection> threadLocal = new ThreadLocal();
    /* 原来：直接从数据池中获取连接 现在：1. 直接获取当前线程绑定的conn 2. 如果连接对象是空的再去连接池中获取conn 在将此conn跟当前对象绑定 */
    public static Connection getConnection () {
        Connection conn = threadLocal.get();
        if (conn == null) {
            conn = database.getConnection();
            threadLocal.set(conn);
        }
        return conn;
    }
    public static void commitAndClose() {
        conn.commit();
        // 解绑conn
        threadLocal.remove();
        conn.close();
    }
    public static void rollbackAndClose() {
        conn.rollback();
        // 解绑conn
        threadLocal.remove();
        conn.close();
    }
}
public class AccountService {
    public boolean transfer(String out, String in, int money) {
        AccountDao dao = new AccountDao();
        Connection conn = null;
        try {
            synchronized (AccountService.class) {
                // 从数据库获取conn并存入当前线程
                // 其他线程到来会从连接池获取新conn
                conn = JdbcUtils.getConnection();
                conn.setAutoCommit(false);
                dao.out(out, money, conn);
                dao.in(in, money, conn);
                JdbcUtils.commitAndClose();
                return true;
            }
        } catch (Exception e) {
            JdbcUtils.rollbackAndClose();
            return false;
        }
    }
}

public class AccountDao {
    // 转出
    public void out(String out, int money) {
        // 从当前线程获取conn
        Connection conn = JdbcUtils.getConnection();
        String sql = "...";
        PreparedStatement pstm = conn.prepareStatement(sql);
        // ...
        pstm.executeUpdate();
    }
    // 转入
    public void in(String in, int money) {
        // 从当前线程获取conn
        Connection conn = JdbcUtils.getConnection();
        String sql = "...";
        PreparedStatement pstm = conn.prepareStatement(sql);
        // ...
        pstm.executeUpdate();
    }
}

ThreadLocal源码

public class ThreadLocal {
    public void set(T value) {
        ThreadLocalMap map = 
            Thread.currentThread().threadLocals;
        if (map != null) {
            map.set(this, value);
        }
    }
    public T get() {
        ThreadLocalMap map = 
            Thread.currentThread().threadLocals;
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                T result = e.value;
                return result;
            }
        }
    }
    public void remove() {
        ThreadLocalMap map = 
            Thread.currentThread().threadLocals;
        if (map != null) {
            m.remove(this);
        }
    }
}