使用try-with-resource的輸入輸出流自動關閉

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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：

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：

try (
    FileReader fr = new FileReader(fileName);
    BufferedReader br = new BufferedReader(fr)
  ) {
    return br.readLine();
}catch (Exception e) {
    log.error("error:{}", e);
}

或者是：

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瞭嗎？

帶著這個問題看看源代碼，發現

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. 從註釋中我們可以發現，實現瞭AutoCloseable並在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。

註意：

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後才運行。

使用try-with-resource需要註意的地方

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：

 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，為瞭防止在try代碼塊中出現異常後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之後上面的代碼就可以簡化成下面的寫法：

  File file = new File("d:/tmp/1.txt");
  try(FileInputStream fis = new FileInputStream(file);) {
   fis.read();
  } catch (IOException e) {
   e.printStackTrace();
  }finally{
  }

可以看出是簡化瞭不少，之所以稱之為語法糖，是因為編譯成class文件後實際的代碼就不是這樣的瞭，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進行反編譯後是下面這樣的

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();
  }

好瞭，上面已經引入今天的主題，try-with-resource，但是仍然有需要註意的地方。

比如下面的代碼：

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，由於MyResource的close方法中出現瞭異常，此時創建的MyResource1就不會被關閉，從而出現資源泄露情況，為瞭規避這個問題，為瞭規避這個問題，我們需要創建的實現AutoCloseable接口的對象單獨創建。

如下面所示：

  try(
    MyResource1 res= new MyResource1();
    MyResource r = new MyResource(res)){
   Integer a = null ;
   a.toString();
  }

以上為個人經驗，希望能給大傢一個參考，也希望大傢多多支持WalkonNet。

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