使用try-with-resources优雅关闭资源-白红宇

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();            }        }    }}复制代码

JDK7及以后关闭资源的正确姿势：try-with-resource Resource的定义：所有实现了接口（其中，它包括实现了的所有对象），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");    }}复制代码

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();        }    }}复制代码

打印结果：

helloResource is closed复制代码

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");    }}复制代码

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();        }    }}复制代码

打印结果：

helloResource say helloResource2 is closedResource is closed复制代码

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();        }    }}复制代码

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");    }}复制代码

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();            }        }    }}复制代码

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)复制代码

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();        }    }}复制代码

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复制代码

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

总结怎么样，是不是很简单呢，如果学会了话