58 - 模板模式

2021-09-27  本文已影响0人  舍是境界

本文来学习另外一种行为型设计模式，模板模式。模板模式主要是用来解决复用和扩展两个问题

模板模式的原理与实现

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.

模板方法模式在一个方法中定义一个算法骨架，并将某些步骤推迟到子类中实现。
模板方法模式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。

public abstract class AbstractClass {
  public final void templateMethod() {
    //...
    method1();
    //...
    method2();
    //...
  }
  
  protected abstract void method1();
  protected abstract void method2();
}
public class ConcreteClass1 extends AbstractClass {
  @Override
  protected void method1() {
    //...
  }
  
  @Override
  protected void method2() {
    //...
  }
}
public class ConcreteClass2 extends AbstractClass {
  @Override
  protected void method1() {
    //...
  }
  
  @Override
  protected void method2() {
    //...
  }
}
AbstractClass demo = ConcreteClass1();
demo.templateMethod();

模板模式作用一：复用

  1. Java InputStream
public abstract class InputStream implements Closeable {
  //...省略其他代码...
  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 {
  //...省略其他代码...
  
  @Override
  public synchronized int read() {
    return (pos < count) ? (buf[pos++] & 0xff) : -1;
  }
}
  1. Java 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();
}

模板模式作用二：扩展

  1. Java Servlet
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>
    <servlet-name>HelloServlet</servlet-name>
    <servlet-class>com.xzg.cd.HelloServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>HelloServlet</servlet-name>
    <url-pattern>/hello</url-pattern>
</servlet-mapping>

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);
    }
}
  1. JUnit TestCase
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 {
  }
}

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