你还在使用 try-catch-finally 关闭资源？

怼烎@ 2021-08-30 12:48 327阅读 0赞

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作者：何甜甜在吗  
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");
        }
    
        @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，][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！

![format_png][]

###  ###

参考资料

\[1\]

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

\[2\]

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

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[Java]: https://blog.csdn.net/qq_37217713/article/details/101445094
[JDK7]: http://mp.weixin.qq.com/s?__biz=MzI3ODcxMzQzMw%3D%3D&chksm=eb53828cdc240b9a4f7c8dafd51d5e1191dd761c25091fd565c1f9d2ecdfeae8a24e58e84848&idx=1&mid=2247485370&scene=21&sn=8349ed975ce56265b8ef7223d3324f93#wechat_redirect
[bug]: http://mp.weixin.qq.com/s?__biz=MzI3ODcxMzQzMw%3D%3D&chksm=eb5071a0dc27f8b670680a7465e0f7776a80b666e804e5d893d7fbf69caf7723b0c160e0bdc4&idx=1&mid=2247496854&scene=21&sn=b72ea3f5f40726eab42f738b59f9236f#wechat_redirect
[format_png]: /images/20210730/3394e2e83ce145cca870cc76da52f64a.png
[format_png 1]: /images/20210730/6d84569ec60a4bb6917058a0834a5006.png