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使用try-with-resource的输入输出流自动关闭

时间:2021-11-01 10:50     来源/作者:wunlie

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

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