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"); } @Override public 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"); } @Override public 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。最近写的这篇：写了个全局变量的bug，也推荐看下。
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"); } @Override public 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 Exception at 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 Exception at 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 Exception at 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
