使用try-with-resources优雅关闭资源

代码必定得写的优雅一点

你还在使用try-catch-finally关闭资源吗，若是是，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以前资源的关闭姿式：

/**
 * jdk7之前关闭流的方式
 *
 * @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();
            }
        }
    }
}
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JDK7及之后关闭资源的正确姿式：try-with-resource Resource的定义：全部实现了 java.lang.AutoCloseable 接口（其中，它包括实现了 java.io.Closeable 的全部对象），能够使用做为资源。 简单Demo进行证明： 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");
    }
}
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测试类CloseResourceIn7.java

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

hello
Resource is closed
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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");
    }
}
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测试类CloseResourceIn7.java

/**
 * jdk7及之后关闭流的方式
 *
 * @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();
        }
    }
}
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打印结果：

hello
Resource say hello
Resource2 is closed
Resource is closed
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即便资源不少，代码也能够写的很简洁，若是用jdk7以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();
        }

    }
}
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能够发现编译之后生成了try-catch-finally语句块 finally中的var2.addSuppressed(var11);是否是有疑问🤔️ 其实这么作是为了处理异常屏蔽的 咱们将代码修改一下 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");
    }
}
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两个方法里面都抛出异常

测试类CloseResourceIn7.java

/**
 * jdk7及之后关闭流的方式
 *
 * @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();
            }
        }
    }
}
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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)
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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必定的困难 咱们换成try-with-resource方法实现CloseResourceIn7.java

/**
 * jdk7及之后关闭流的方式
 *
 * @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();
        }

    }
}
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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
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能够发现，异常信息中多了一个Suppressed的提示，告诉咱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，是否是很简单呢，若是学会了话