try-with-resource的輸入輸出流自動關閉
最近在做代碼審核的時候，審核工具提示我將 try-catch-finally 給替換掉，而且根據公司相關要求，該提示的級別還不低，不改不予通過。
先看看代碼吧：
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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); } }} |
審核工具給出的意見是 替換為：
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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);} |
對比代碼，不難發現，輸入輸出流的關閉存在著差異。難道輸入輸出流不用關閉了嗎？
帶著這個問題看看源代碼，發現
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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 顧名思義， 自動關閉流. 從注釋中我們可以發現，實現了AutoCloseable并在try()中聲明的對象，當try-with-resource代碼塊執行完的時候，會自動調用close()方法。
注意：
一個 try-with-resources 語句可以像普通的 try 語句那樣有 catch 和 finally 塊。在try-with-resources 語句中， 任意的 catch 或者 finally 塊都是在聲明的資源被關閉以后才運行。
使用try-with-resource需要注意的地方
try-with-resource是JDK7引入的語法糖，可以簡化Autocloseable資源類的關閉過程，
比如JDK7以前下面的代碼：
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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(); } } } |
上面是一段讀取文件內容的示意代碼，為了防止在try代碼塊中出現異常后導致的資源泄露問題，在finally代碼塊中一般處理資源的關閉事項。
JDK之后上面的代碼就可以簡化成下面的寫法：
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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{} |
可以看出是簡化了不少，之所以稱之為語法糖，是因為編譯成class文件后實際的代碼就不是這樣的了，編譯過程中會自動添加資源的關閉處理。
上面的代碼編譯出的class文件使用javap進行反編譯后是下面這樣的
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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，上面已經引入今天的主題，try-with-resource，但是仍然有需要注意的地方。
比如下面的代碼：
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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自動關閉"); Integer a = null; a.toString(); this.res.close(); } } private static class MyResource1 implements AutoCloseable{ @Override public void close() throws Exception { System.out.println("MyResource1自動關閉"); } } @Test public void test() throws Exception{ try( MyResource r = new MyResource(new MyResource1())){ Integer a = null ; a.toString(); } } |
執行上面的代碼，由于MyResource的close方法中出現了異常，此時創建的MyResource1就不會被關閉，從而出現資源泄露情況，為了規避這個問題，為了規避這個問題，我們需要創建的實現AutoCloseable接口的對象單獨創建。
如下面所示：
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try( MyResource1 res= new MyResource1(); MyResource r = new MyResource(res)){ Integer a = null ; a.toString();} |
以上為個人經驗，希望能給大家一個參考，也希望大家多多支持服務器之家。
原文鏈接：https://blog.csdn.net/wangjie_19920912/article/details/69501883
