try-with-resource的输入输出流自动关闭
最近在做代码审核的时候，审核工具提示我将 try-catch-finally 给替换掉，而且根据公司相关要求，该提示的级别还不低，不改不予通过。
先看看代码吧：
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FileReader fr = null; BufferedReader br = null;try { fr = new FileReader(fileName); br = new BufferedReader(fr); return br.readLine();} catch (Exception e) { log.error("error:{}", e);} finally { if (br != null) { try { br.close(); } catch(IOException e){ log.error("error:{}", e); } } if (fr != null ) { try { br.close(); } catch(IOException e){ log.error("error:{}", e); } }} |
审核工具给出的意见是 替换为：
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try ( FileReader fr = new FileReader(fileName); BufferedReader br = new BufferedReader(fr) ) { return br.readLine();}catch (Exception e) { log.error("error:{}", e);} |
或者是：
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try ( BufferedReader br = new BufferedReader(new FileReader(fileName)) ) { // no need to name intermediate resources if you don't want to return br.readLine();}catch (Exception e) { log.error("error:{}", e);} |
对比代码，不难发现，输入输出流的关闭存在着差异。难道输入输出流不用关闭了吗？
带着这个问题看看源代码，发现
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public class FileInputStream extends InputStream{}public abstract class InputStream implements Closeable {}/** * A {@code Closeable} is a source or destination of data that can be closed. * The close method is invoked to release resources that the object is * holding (such as open files). * * @since 1.5 */public interface Closeable extends AutoCloseable {}/** * An object that may hold resources (such as file or socket handles) * until it is closed. The {@link #close()} method of an {@code AutoCloseable} * object is called automatically when exiting a {@code * try}-with-resources block for which the object has been declared in * the resource specification header. This construction ensures prompt * release, avoiding resource exhaustion exceptions and errors that * may otherwise occur. * * @apiNote * <p>It is possible, and in fact common, for a base class to * implement AutoCloseable even though not all of its subclasses or * instances will hold releasable resources. For code that must operate * in complete generality, or when it is known that the {@code AutoCloseable} * instance requires resource release, it is recommended to use {@code * try}-with-resources constructions. However, when using facilities such as * {@link java.util.stream.Stream} that support both I/O-based and * non-I/O-based forms, {@code try}-with-resources blocks are in * general unnecessary when using non-I/O-based forms. * * @author Josh Bloch * @since 1.7 */public interface AutoCloseable {} |
AutoCloseable 顾名思义， 自动关闭流. 从注释中我们可以发现，实现了AutoCloseable并在try()中声明的对象，当try-with-resource代码块执行完的时候，会自动调用close()方法。
注意：
一个 try-with-resources 语句可以像普通的 try 语句那样有 catch 和 finally 块。在try-with-resources 语句中， 任意的 catch 或者 finally 块都是在声明的资源被关闭以后才运行。
使用try-with-resource需要注意的地方
try-with-resource是JDK7引入的语法糖，可以简化Autocloseable资源类的关闭过程，
比如JDK7以前下面的代码：
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File file = new File("d:/tmp/1.txt"); FileInputStream fis = null; try { fis = new FileInputStream(file); xxxxx xxxxx } catch (IOException e) { e.printStackTrace(); }finally{ if(fis != null){ try { fis.close(); } catch (IOException e) { e.printStackTrace(); } } } |
上面是一段读取文件内容的示意代码，为了防止在try代码块中出现异常后导致的资源泄露问题，在finally代码块中一般处理资源的关闭事项。
JDK之后上面的代码就可以简化成下面的写法：
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File file = new File("d:/tmp/1.txt");try(FileInputStream fis = new FileInputStream(file);) { fis.read();} catch (IOException e) { e.printStackTrace();}finally{} |
可以看出是简化了不少，之所以称之为语法糖，是因为编译成class文件后实际的代码就不是这样的了，编译过程中会自动添加资源的关闭处理。
上面的代码编译出的class文件使用javap进行反编译后是下面这样的
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File file = new File("d:/tmp/1.txt"); try { Throwable var2 = null; Object var3 = null; try { FileInputStream fis = new FileInputStream(file); xxx xxxx } catch (Throwable var12) { if (var2 == null) { var2 = var12; } else if (var2 != var12) { var2.addSuppressed(var12); } throw var2; } } catch (IOException var13) { var13.printStackTrace(); } |
好了，上面已经引入今天的主题，try-with-resource，但是仍然有需要注意的地方。
比如下面的代码：
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private static class MyResource implements AutoCloseable{ private MyResource1 res; public MyResource(MyResource1 res){ this.res = res; } @Override public void close() throws Exception { System.out.println("MyResource自动关闭"); Integer a = null; a.toString(); this.res.close(); } } private static class MyResource1 implements AutoCloseable{ @Override public void close() throws Exception { System.out.println("MyResource1自动关闭"); } } @Test public void test() throws Exception{ try( MyResource r = new MyResource(new MyResource1())){ Integer a = null ; a.toString(); } } |
执行上面的代码，由于MyResource的close方法中出现了异常，此时创建的MyResource1就不会被关闭，从而出现资源泄露情况，为了规避这个问题，为了规避这个问题，我们需要创建的实现AutoCloseable接口的对象单独创建。
如下面所示：
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try( MyResource1 res= new MyResource1(); MyResource r = new MyResource(res)){ Integer a = null ; a.toString();} |
以上为个人经验，希望能给大家一个参考，也希望大家多多支持服务器之家。
原文链接：https://blog.csdn.net/wangjie_19920912/article/details/69501883
