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使用try-with-resource的输入输出流自动关闭

try-with-resource的输入输出流自动关闭 【使用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 toreturn 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 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); xxxxxxxxxx} 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); xxxxxxx} 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; }@Overridepublic 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{ @Overridepublic 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脚本之家。