execute around 模式

在写代码的时候，常常会遇到这样的情况：每次在执行一段代码的时候，都需要先执行一些代码；执行完之后，再执行另一些代码。比如在读写文件的时候，我们真正care的逻辑是"读写",至于文件流的初始化、打开、关闭、异常处理等都不是我们关心的。但是，每次在"读写"之前，我们需要执行打开操作，"读写"之后，需要执行关闭操作。这个时候就可以使用"execute around 模式"来提炼出"读写"前后的公共代码，从而把心思放在"读写"的逻辑上。

类图：

代码：

/**
 * 
 * Interface for specifying what to do with the file resource.
 *
 */
public interface FileWriterAction {

  void writeFile(FileWriter writer) throws IOException;

}

/**
 * 
 * SimpleFileWriter handles opening and closing file for the user. The user only has to specify what
 * to do with the file resource through {@link FileWriterAction} parameter.
 *
 */
public class SimpleFileWriter {

  /**
   * Constructor
   */
  public SimpleFileWriter(String filename, FileWriterAction action) throws IOException {
    FileWriter writer = new FileWriter(filename); // 读写前的操作
    try {
      action.writeFile(writer); // 读写
    } finally {
      writer.close(); // 读写后的操作
    }
  }
}

/**
 * The Execute Around idiom specifies some code to be executed before and after a method. Typically
 * the idiom is used when the API has methods to be executed in pairs, such as resource
 * allocation/deallocation or lock acquisition/release.
 * <p>
 * In this example, we have {@link SimpleFileWriter} class that opens and closes the file for the
 * user. The user specifies only what to do with the file by providing the {@link FileWriterAction}
 * implementation.
 *
 */
public class App {

  /**
   * Program entry point
   */
  public static void main(String[] args) throws IOException {

    new SimpleFileWriter("testfile.txt", new FileWriterAction() {

      @Override
      public void writeFile(FileWriter writer) throws IOException {
        // 真正关心的"读写"逻辑
        writer.write("Hello");
        writer.append(" ");
        writer.append("there!");
      }
    });
  }
}