exceptions - Try With Resources

最新推荐文章于 2023-08-28 10:33:32 发布

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本文介绍Java7引入的Try-With-Resources语法，显著简化了资源管理过程，尤其是在处理文件输入流时。通过实现java.lang.AutoCloseable接口，确保资源在try块退出时自动关闭，避免了复杂的finally块。

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// exceptions/MessyExceptions.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.

import java.io.*;

public class MessyExceptions {
  public static void main(String[] args) {
    InputStream in = null;
    try {
      in = new FileInputStream(new File("MessyExceptions.java"));
      int contents = in.read();
      // Process contents
    } catch (IOException e) {
      // Handle the error
    } finally {
      if (in != null) {
        try {
          in.close();
        } catch (IOException e) {
          // Handle the close() error
        }
      }
    }
  }
}

When the finally clause has its own try block, it feels like things have become overcomplicated. Fortunately Java 7 introduces the try-with-resources syntax, which cleans up the above code remarkably well:

// exceptions/TryWithResources.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
import java.io.*;

public class TryWithResources {
  public static void main(String[] args) {
    try(
      InputStream in = new FileInputStream(
        new File("TryWithResources.java"))
    ) {
      int contents = in.read();
      // Process contents
    } catch(IOException e) {
      // Handle the error
    }
  }
}

Before Java 7, a try was always followed by a {, but now it can be followed by a parenthesized definition—here our creation of the FileInputStream object. The part within parentheses is called the resource specification header. Now in is available throughout the rest of the try block. More importantly, no matter how we exit the try block (normally or via exception), the equivalent of the previous finally clause is executed, but without writing that messy and tricky code. This is an important improvement.

It works by implementing the java.lang.Autocoseable interface(since 1.7), which has a single method, close().

references:

1. On Java 8 - Bruce Eckel

2. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/exceptions/MessyExceptions.java

3. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/exceptions/TryWithResources.java