ThreadLocal的应用

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ThreadLocal 内存泄露问题

ThreadLocal本身不存储值,他只作为一个key。真正存值的是thread

public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    return setInitialValue();
}

ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;//t.threadLocals是ThreadLocal.ThreadLocalMap类型
}

也就是说，threadlocal的生命周期是一个普通的变量的生命周期，而threadLocal中存放的数据的生命周期是当前thread的生命周期（thread不销毁一直都存在）。如果遇到thread不销毁的情况，比如使用线程池，不断创建新的threadLocal作为key 向 Thread.threadLocals中插入对象，而之前的对象也不会被回收，因为有Thread.threadLocals的引用。这样会造成内存泄露。所以需要在使用完成后，手动从threadLocal中删除数据。

ThreadLocal 应用

通常我们写一个工具类都是无状态的。例如下面写一个db的工具类。

public class DatabaseHelper1 {

    private static final Logger LOGGER = LoggerFactory.getLogger(DatabaseHelper.class);
    private static final String DRIVER;
    private static final String URL;
    private static final String USERNAME;
    private static final String PASSWORD;

    static {
        Properties conf = PropsUtil.loadProps("config.properties");
        String driver = conf.getProperty("jdbc.driver");
        String url = conf.getProperty("jdbc.url");
        String username = conf.getProperty("jdbc.username");
        String password = conf.getProperty("jdbc.password");

        DRIVER = driver;
        URL = url;
        USERNAME = username;
        PASSWORD = password;
    }

  //示例代码
    public static Connection getConnection() throws SQLException {
        return DriverManager.getConnection(URL, USERNAME, PASSWORD);
    }

  //示例代码
    public static <T>List<T> queryEntityList(Connection connection, Class<T> entityClass, String sql, Object... params) {
        return new ArrayList<>();
    }
}

这样的话service调用就比较麻烦

    public List<Customer> getCustomerList1() throws SQLException {
        Connection connection = DatabaseHelper1.getConnection();
        String sql = "select * from customer";
        return DatabaseHelper1.queryEntityList(connection,Customer.class, sql);
    }

能不能让DatabaseHelper工具类有记忆功能？不用每次service都是先取到connection再做其他操作。threadLocal可以帮我们实现。

public class DatabaseHelper {
      private static ThreadLocal<Connection> threadConnection = new ThreadLocal<>();

      public static Connection getConnection() {
        Connection connection = threadConnection.get();
        if (connection == null) {
            try {
                connection = DriverManager.getConnection(URL, USERNAME, PASSWORD);
            } catch (SQLException e) {
                LOGGER.error("get connection fail");
            } finally {
                threadConnection.set(connection);
            }
        }
        return connection;
    }
  
      public static void closeConnetion(Connection connection) {
        if (connection != null) {
            try {
                connection.close();
            } catch (SQLException e) {
                LOGGER.error("close connection fail");
            } finally {
                threadConnection.remove();
            }
        }
    }
  
      public static <T> List<T> queryEntityList(Class<T> entityClass, String sql, Object... params) {
        List<T> entityList = new ArrayList<>();
        Connection connection = getConnection();
        try {
            entityList = QUERY_RUNNER.query(connection, sql, new BeanListHandler<T>(entityClass), params);
        } catch (SQLException e) {
            LOGGER.error("queryEntity fail", e);
        } finally {
            closeConnetion(connection);
        }
        return entityList;
    }
}

通过threadLocal，使service调用变的简单

public List<Customer> getCustomerList() {
   	String sql = "select * from customer";
  	return DatabaseHelper.queryEntityList(Customer.class, sql);
}

但是还记得上面提到的threadlocal内存泄露么，在每次使用完threadLocal记得将其内容删除。就是closeConnection()中的threadConnection.remove();

转载于:https://my.oschina.net/u/1245414/blog/1533861