try - with - resources详解

概述

try-with-resources自JDK7引入，在JDK9中进行了改进，使得用户可以更加方便、简洁的使用try-with-resources。

JDK7之前资源需要手动关闭

下面是一个常见的文件资源关闭的示例：

BufferedWriter writer = null;
try {
   
    writer = Files.newBufferedWriter(file, charset);
    writer.write(s, 0, s.length());
} catch (IOException x) {
   
    System.err.format("IOException: %s%n", x);
} finally {
   
    if (writer != null) writer.close();
}

因此在JDK7之前，必须要牢记在finally中执行关闭资源的方法，否则随着程序不断运行，资源泄露将会累计成重大的生产事故，然而如果你同时打开多个资源，那么将会出现噩梦般的场景。

public class Demo {
   
    public static void main(String[] args) {
   
        BufferedInputStream bin = null;
        BufferedOutputStream bout = null;
        try {
   
            bin = new BufferedInputStream(new FileInputStream(new File("test.txt")));
            bout = new BufferedOutputStream(new FileOutputStream(new File("out.txt")));
            int b;
            while ((b = bin.read()) != -1) {
   
                bout.write(b);
            }
        }
        catch (IOException e) {
   
            e.printStackTrace();
        }
        finally {
   
            if (bin != null) {
   
                try {
   
                    bin.close();
                }
                catch (IOException e) {
   
                    e.printStackTrace();
                }
                finally {
   
                    if (bout != null) {
   
                        try {
   
                            bout.close();
                        }
                        catch (IOException e) {
   
                            e.printStackTrace();
                        }
                    }
                }
            }
        }
    }
}

关闭资源的代码竟然比业务代码还要多！！！这是因为，我们不仅需要关闭BufferedInputStream，还需要保证如果关闭BufferedInputStream时出现了异常， BufferedOutputStream也要能被正确地关闭。所以我们不得不借助finally中嵌套finally大法。可以想到，打开的资源越多，finally中嵌套的将会越深！！！

JDK7使用try-with-resources语法糖

我们可以利用Java 1.7中新增的try-with-resource语法糖来打开资源，而无需码农们自己书写资源来关闭代码。

public class TryWithResource {
   

    public static void main(String[] args) {
   
        try (BufferedInputStream bin = new BufferedInputStream(new FileInputStream(new File("test.txt")));
             BufferedOutputStream bout = new BufferedOutputStream(new FileOutputStream(new File("out.txt")))) {
   
            int b