使用try

使用try

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

*

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 releasehttp://, 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 BlocuDonWh

* @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 FileInputSthttp://ream(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();

}

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