Template Method

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Template Method
概念
模板方法：Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm’s structure. 在一个操作中定义一个算法骨架，并将某些步骤推迟到子类实现。可以让子类在不改变算法整体结构的情况下，重新定义算法中的某些步骤。
算法可以理解为业务逻辑。
模板方法的作用：
复用：子类都可以复用父类中模板方法定义的流程代码。如 InputStream、OutputStream、Reader、Writer，AbstractList 等。
扩展：更多指框架的扩展性，类似控制反转，可以让用户不修改框架源码的情况下，定制化框架的功能。如 Java Servlet、JUnit 等。
示例
public abstract class AbstractClass {
  public final void templateMethod() {
    //...
    method1();
    //...
    method2();
    //...
  }
  
  protected abstract void method1();
  protected abstract void method2();
}
​
public class ConcreteClass extends AbstractClass {
  @Override
  protected void method1() {}
  
  @Override
  protected void method2() {}
}
​
AbstractClass demo = ConcreteClass();
demo.templateMethod();
模板方法用 final 修饰，避免子类重写；扩展点用 abstract 修饰，强迫子类实现。不过这两点都不是必须的。
源码
InputStream
​
public abstract class InputStream implements Closeable {
  
  public int read(byte b[], int off, int len) throws IOException {
    if (b == null) {
      throw new NullPointerException();
    } else if (off < 0 || len < 0 || len > b.length - off) {
      throw new IndexOutOfBoundsException();
    } else if (len == 0) {
      return 0;
    }
​
    int c = read();
    if (c == -1) {
      return -1;
    }
    b[off] = (byte)c;
​
    int i = 1;
    try {
      for (; i < len ; i++) {
        c = read();
        if (c == -1) {
          break;
        }
        b[off + i] = (byte)c;
      }
    } catch (IOException ee) {
    }
    return i;
  }
  
  public abstract int read() throws IOException;
}
​
public class ByteArrayInputStream extends InputStream {
​
  @Override
  public synchronized int read() {
    return (pos < count) ? (buf[pos++] & 0xff) : -1;
  }
}
AbstractList
public boolean addAll(int index, Collection<? extends E> c) {
    rangeCheckForAdd(index);
    boolean modified = false;
    for (E e : c) {
        add(index++, e);
        modified = true;
    }
    return modified;
}
​
public void add(int index, E element) {
    throw new UnsupportedOperationException();
}
Servlet
用户实现的扩展点：
public class HelloServlet extends HttpServlet {
  @Override
  protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    this.doPost(req, resp);
  }
  
  @Override
  protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    resp.getWriter().write("Hello World.");
  }
}
Servlet 容器收到请求后，会找到相应的 Servlet，并调用它的 service 方法。框架的模板方法：
public void service(ServletRequest req, ServletResponse res)
    throws ServletException, IOException
{
    HttpServletRequest  request;
    HttpServletResponse response;
    if (!(req instanceof HttpServletRequest &&
            res instanceof HttpServletResponse)) {
        throw new ServletException("non-HTTP request or response");
    }
    request = (HttpServletRequest) req;
    response = (HttpServletResponse) res;
    service(request, response);
}
​
protected void service(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
{
    String method = req.getMethod();
    if (method.equals(METHOD_GET)) {
        long lastModified = getLastModified(req);
        if (lastModified == -1) {
            // servlet doesn't support if-modified-since, no reason
            // to go through further expensive logic
            doGet(req, resp);
        } else {
            long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
            if (ifModifiedSince < lastModified) {
                // If the servlet mod time is later, call doGet()
                // Round down to the nearest second for a proper compare
                // A ifModifiedSince of -1 will always be less
                maybeSetLastModified(resp, lastModified);
                doGet(req, resp);
            } else {
                resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
            }
        }
    } else if (method.equals(METHOD_HEAD)) {
        long lastModified = getLastModified(req);
        maybeSetLastModified(resp, lastModified);
        doHead(req, resp);
    } else if (method.equals(METHOD_POST)) {
        doPost(req, resp);
    } else if (method.equals(METHOD_PUT)) {
        doPut(req, resp);
    } else if (method.equals(METHOD_DELETE)) {
        doDelete(req, resp);
    } else if (method.equals(METHOD_OPTIONS)) {
        doOptions(req,resp);
    } else if (method.equals(METHOD_TRACE)) {
        doTrace(req,resp);
    } else {
        String errMsg = lStrings.getString("http.method_not_implemented");
        Object[] errArgs = new Object[1];
        errArgs[0] = method;
        errMsg = MessageFormat.format(errMsg, errArgs);
        resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
    }
}
JUnit
JUnit 提供了一些扩展点，如 setUp(), tearDown() 等。
public abstract class TestCase extends Assert implements Test {
  public void runBare() throws Throwable {
    Throwable exception = null;
    setUp();
    try {
      runTest();
    } catch (Throwable running) {
      exception = running;
    } finally {
      try {
        tearDown();
      } catch (Throwable tearingDown) {
        if (exception == null) exception = tearingDown;
      }
    }
    if (exception != null) throw exception;
  }
  
  /**
  * Sets up the fixture, for example, open a network connection.
  * This method is called before a test is executed.
  */
  protected void setUp() throws Exception {
  }
​
  /**
  * Tears down the fixture, for example, close a network connection.
  * This method is called after a test is executed.
  */
  protected void tearDown() throws Exception {
  }
}
Callback
回调与模板模式一样也具有复用和扩展的功能。
原理
A 类事先注册某个函数 F 到 B 类，A 类在调用 B 类的 P 函数时，B 类反过来调用 A 类注册给 B 类的 F 函数，F 就叫做回调函数。
回调不仅可以用在代码设计上，还能用在高层次的架构设计上。比如用户向第三方支付系统发起支付，一般不会等到支付结果返回，而是注册回调函数（URL）给第三方支付系统，支付系统执行成功后，将结果通过回调接口返回给用户。
回调有同步回调和异步回调，同步回调更像模板模式，异步回调更像观察者模式。
示例
public interface ICallback {
  void methodToCallback();
}
​
public class BClass {
  public void process(ICallback callback) {
    //...
    callback.methodToCallback();
    //...
  }
}
​
public class AClass {
  public static void main(String[] args) {
    BClass b = new BClass();
    b.process(new ICallback() { //回调对象
      @Override
      public void methodToCallback() {
        System.out.println("Call back me.");
      }
    });
  }
}
JdbcTemplate
Spring 提供很多 Template 类，但它们并非基于模板方法实现，而是基于 Callback 实现。
如果直接写 JDBC 代码会很冗余，比如：
public class JdbcDemo {
  public User queryUser(long id) {
    Connection conn = null;
    Statement stmt = null;
    try {
      //1.加载驱动
      Class.forName("com.mysql.jdbc.Driver");
      conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/demo", "xzg", "xzg");
​
      //2.创建statement类对象，用来执行SQL语句
      stmt = conn.createStatement();
​
      //3.ResultSet类，用来存放获取的结果集
      String sql = "select * from user where id=" + id;
      ResultSet resultSet = stmt.executeQuery(sql);
​
      String eid = null, ename = null, price = null;
​
      while (resultSet.next()) {
        User user = new User();
        user.setId(resultSet.getLong("id"));
        user.setName(resultSet.getString("name"));
        user.setTelephone(resultSet.getString("telephone"));
        return user;
      }
    } catch (ClassNotFoundException e) {
      // TODO: log...
    } catch (SQLException e) {
      // TODO: log...
    } finally {
      if (conn != null)
        try {
          conn.close();
        } catch (SQLException e) {
          // TODO: log...
        }
      if (stmt != null)
        try {
          stmt.close();
        } catch (SQLException e) {
          // TODO: log...
        }
    }
    return null;
  }
}
加载驱动、创建连接、关闭连接等很多操作与业务无关，这些流程大多可以复用，所以 Spring 提供了 JdbcTemplate，用户代码可以变得非常简洁：
public class JdbcTemplateDemo {
  private JdbcTemplate jdbcTemplate;
​
  public User queryUser(long id) {
    String sql = "select * from user where id="+id;
    return jdbcTemplate.query(sql, new UserRowMapper()).get(0);
  }
​
  class UserRowMapper implements RowMapper<User> {
    public User mapRow(ResultSet rs, int rowNum) throws SQLException {
      User user = new User();
      user.setId(rs.getLong("id"));
      user.setName(rs.getString("name"));
      user.setTelephone(rs.getString("telephone"));
      return user;
    }
  }
}
JdbcTemplate 核心的逻辑如下：
@Override
public <T> List<T> query(String sql, RowMapper<T> rowMapper) throws DataAccessException {
 return query(sql, new RowMapperResultSetExtractor<T>(rowMapper));
}
​
@Override
public <T> T query(final String sql, final ResultSetExtractor<T> rse) throws DataAccessException {
 class QueryStatementCallback implements StatementCallback<T>, SqlProvider {
  @Override
  public T doInStatement(Statement stmt) throws SQLException {
   ResultSet rs = null;
   try {
    rs = stmt.executeQuery(sql);
    ResultSet rsToUse = rs;
    if (nativeJdbcExtractor != null) {
     rsToUse = nativeJdbcExtractor.getNativeResultSet(rs);
    }
    return rse.extractData(rsToUse);
   }
   finally {
    JdbcUtils.closeResultSet(rs);
   }
  }
  @Override
  public String getSql() {
   return sql;
  }
 }
​
 return execute(new QueryStatementCallback());
}
​
@Override
public <T> T execute(StatementCallback<T> action) throws DataAccessException {
 Assert.notNull(action, "Callback object must not be null");
​
 Connection con = DataSourceUtils.getConnection(getDataSource());
 Statement stmt = null;
 try {
  Connection conToUse = con;
  if (this.nativeJdbcExtractor != null &&
    this.nativeJdbcExtractor.isNativeConnectionNecessaryForNativeStatements()) {
   conToUse = this.nativeJdbcExtractor.getNativeConnection(con);
  }
  stmt = conToUse.createStatement();
  applyStatementSettings(stmt);
  Statement stmtToUse = stmt;
  if (this.nativeJdbcExtractor != null) {
   stmtToUse = this.nativeJdbcExtractor.getNativeStatement(stmt);
  }
  T result = action.doInStatement(stmtToUse);
  handleWarnings(stmt);
  return result;
 }
 catch (SQLException ex) {
  // Release Connection early, to avoid potential connection pool deadlock
  // in the case when the exception translator hasn't been initialized yet.
  JdbcUtils.closeStatement(stmt);
  stmt = null;
  DataSourceUtils.releaseConnection(con, getDataSource());
  con = null;
  throw getExceptionTranslator().translate("StatementCallback", getSql(action), ex);
 }
 finally {
  JdbcUtils.closeStatement(stmt);
  DataSourceUtils.releaseConnection(con, getDataSource());
 }
}
Hook
Callback 更加注重语法机制的描述，Hook 更加侧重应用的描述。Tomcat 和 JVM 都有shutdown hook，以 JVM 为例，当应用程序关闭时，会调用 ApplicationShutdownHooks 的 runHooks 方法：
public class ShutdownHookDemo {
​
  private static class ShutdownHook extends Thread {
    public void run() {
      System.out.println("I am called during shutting down.");
    }
  }
​
  public static void main(String[] args) {
    Runtime.getRuntime().addShutdownHook(new ShutdownHook());
  }
}
​
​
public class Runtime {
  public void addShutdownHook(Thread hook) {
    SecurityManager sm = System.getSecurityManager();
    if (sm != null) {
      sm.checkPermission(new RuntimePermission("shutdownHooks"));
    }
    ApplicationShutdownHooks.add(hook);
  }
}
​
class ApplicationShutdownHooks {
    /* The set of registered hooks */
    private static IdentityHashMap<Thread, Thread> hooks;
    static {
            hooks = new IdentityHashMap<>();
        } catch (IllegalStateException e) {
            hooks = null;
        }
    }
​
    static synchronized void add(Thread hook) {
        if(hooks == null)
            throw new IllegalStateException("Shutdown in progress");
​
        if (hook.isAlive())
            throw new IllegalArgumentException("Hook already running");
​
        if (hooks.containsKey(hook))
            throw new IllegalArgumentException("Hook previously registered");
​
        hooks.put(hook, hook);
    }
​
    static void runHooks() {
        Collection<Thread> threads;
        synchronized(ApplicationShutdownHooks.class) {
            threads = hooks.keySet();
            hooks = null;
        }
​
        for (Thread hook : threads) {
            hook.start();
        }
        for (Thread hook : threads) {
            while (true) {
                try {
                    hook.join();
                    break;
                } catch (InterruptedException ignored) {
                }
            }
        }
    }
}
比较
同步回调几乎与模板模式一致，都是在一个大的算法骨架中，自由替换其中的个别步骤，起到代码复用和扩展的目的。
实现方式：模板方法基于继承；回调基于组合。
组合优于继承，那么回调相对于模板方法的优势如下：
Java 只支持单继承。
回调可以使用匿名类，不用事先定义类。
如果一个类有多个模板方法，每个类都有对应的抽象方法，那么模板方法需要实现所有的抽象方法；而回调只需要往我们需要的模板方法中注入回调对象即可。
Observer
Strategy
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