try-with-resource输入输出流关闭使用try-with-resource的输入输出流自动关闭

时间:2021-07-31 wunlie 人气:0

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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：

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：

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);
}

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

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：

 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：

  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

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：

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：

  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。

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