你还在使用 try-catch-finally 关闭资源？不太优雅~

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链接：https://juejin.im/post/5b8f9fa05188255c6f1df755

代码一定得写的优雅一点！

你还在使用try-catch-finally关闭资源吗，如果是，那么就有点out了。皮皮甜手把手教你使用JDK7引用的try-with-resource

JDK7之前资源的关闭姿势：

/*** jdk7以前关闭流的方式** @author hetiantian* */
public class CloseResourceBefore7 {private static final String FileName = "file.txt";public static void main(String[] args) throws IOException {FileInputStream inputStream = null;try {inputStream = new FileInputStream(FileName);char c1 = (char) inputStream.read();System.out.println("c1=" + c1);} catch (IOException e) {e.printStackTrace();} finally {if (inputStream != null) {inputStream.close();}}}
}

JDK7及以后关闭资源的正确姿势

try-with-resource Resource的定义：

所有实现了 java.lang.AutoCloseable^[1] 接口（其中，它包括实现了 java.io.Closeable^[2] 的所有对象），可以使用作为资源。简单Demo进行证实：实现java.lang.AutoCloseable接口的Resource类：

/*** 资源类** @author hetiantian* */
public class Resource implements AutoCloseable {public void sayHello() {System.out.println("hello");}@Overridepublic void close() throws Exception {System.out.println("Resource is closed");}
}

测试类CloseResourceIn7.java

/*** jdk7及以后关闭流的方式** @author hetiantian* */
public class CloseResourceIn7 {public static void main(String[] args) {try(Resource resource = new Resource()) {resource.sayHello();} catch (Exception e) {e.printStackTrace();}}
}

打印结果：

hello
Resource is closed

当存在多个打开资源的时候：资源二Resource2.java

/*** 资源2** @author hetiantian* */
public class Resource2 implements AutoCloseable {public void sayhello() {System.out.println("Resource say hello");}@Overridepublic void close() throws Exception {System.out.println("Resource2 is closed");}
}

测试类CloseResourceIn7.java

/*** jdk7及以后关闭流的方式** @author hetiantian* */
public class CloseResourceIn7 {public static void main(String[] args) {try(Resource resource = new Resource(); Resource2 resource2 = new Resource2()) {resource.sayHello();resource2.sayhello();} catch (Exception e) {e.printStackTrace();}}
}

打印结果：

hello
Resource say hello
Resource2 is closed
Resource is closed

即使资源很多，代码也可以写的很简洁，如果用JDK7之前的方式去关闭资源，那么资源越多，用fianl关闭资源时嵌套也就越多。

那么它的底层原理又是怎样的呢，由皮皮甜独家揭秘优雅关闭资源背后的密码秘密

查看编译的class文件CloseResourceIn7.class：

public class CloseResourceIn7 {public CloseResourceIn7() {}public static void main(String[] args) {try {Resource resource = new Resource();Throwable var2 = null;try {resource.sayHello();} catch (Throwable var12) {var2 = var12;throw var12;} finally {if (resource != null) {if (var2 != null) {try {resource.close();} catch (Throwable var11) {var2.addSuppressed(var11);}} else {resource.close();}}}} catch (Exception var14) {var14.printStackTrace();}}
}

可以发现编译以后生成了try-catch-finally语句块 finally中的var2.addSuppressed(var11);

是不是有疑问？其实这么做是为了处理异常屏蔽的，我们将代码修改一下。

资源Resource.java

/*** 资源类** @author hetiantian* */
public class Resource implements AutoCloseable {public void sayHello() throws Exception {throw new Exception("Resource throw Exception");}@Overridepublic void close() throws Exception {throw new Exception("Close method throw Exception");}
}

两个方法里面都抛出异常

测试类CloseResourceIn7.java

/*** jdk7及以后关闭流的方式** @author hetiantian* */
public class CloseResourceIn7 {public static void main(String[] args) {try {errorTest();} catch (Exception e) {e.printStackTrace();}}private static void errorTest() throws Exception {Resource resource = null;try {resource = new Resource();resource.sayHello();}finally {if (resource != null) {resource.close();}}}
}

打印结果：

java.lang.Exception: Close method throw Exceptionat com.shuwen.Resource.close(Resource.java:15)at com.shuwen.CloseResourceIn7.errorTest(CloseResourceIn7.java:27)at com.shuwen.CloseResourceIn7.main(CloseResourceIn7.java:12)

只打印了最后出现的异常【异常屏蔽】这样会给开发人员排查错误带来一定的困难 我们换成try-with-resource方法实现CloseResourceIn7.java

/*** jdk7及以后关闭流的方式** @author hetiantian* */
public class CloseResourceIn7 {public static void main(String[] args) {try {errorTest();} catch (Exception e) {e.printStackTrace();}}private static void errorTest() throws Exception {try(Resource resource = new Resource()) {resource.sayHello();}}
}

打印信息：

java.lang.Exception: Resource throw Exceptionat com.shuwen.Resource.sayHello(Resource.java:10)at com.shuwen.CloseResourceIn7.errorTest(CloseResourceIn7.java:20)at com.shuwen.CloseResourceIn7.main(CloseResourceIn7.java:12)Suppressed: java.lang.Exception: Close method throw Exceptionat com.shuwen.Resource.close(Resource.java:15)at com.shuwen.CloseResourceIn7.errorTest(CloseResourceIn7.java:21)... 1 more

可以发现，异常信息中多了一个Suppressed的提示，告诉我们这个异常其实由两个异常组成，Close method throw Exception这个异常是被Suppressed【屏蔽】的异常

怎么样，是不是很简单呢，如果学会了话来个在看吧！

参考资料

[1]

java.lang.AutoCloseable: http://docs.oracle.com/javase/8/docs/api/java/lang/AutoCloseable.html

java.io.Closeable: http://docs.oracle.com/javase/8/docs/api/java/io/Closeable.html

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