你还在使用 try-catch-finally 关闭资源？

作者：何甜甜在吗
https://juejin.im/post/5b8f9f...

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

怎么样，是不是很简单呢，如果学会了话来个在看吧！