你還在使用 try-catch-finally 關閉資源？

代碼一定得寫的優雅一點！

你還在使用try-catch-finally關閉資源嗎，如果是，那麼就有點out了。皮皮甜手把手教你使用

JDK7

引用的try-with-resource

JDK7之前資源的關閉姿勢：

/**  
 * jdk7以前關閉流的方式  
 *  
 * @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及以後關閉資源的正确姿勢

try-with-resource Resource的定義：

所有實作了 java.lang.AutoCloseable[1] 接口（其中，它包括實作了 java.io.Closeable[2] 的所有對象），可以使用作為資源。簡單Demo進行證明：實作java.lang.AutoCloseable接口的Resource類：

/**  
 * 資源類  
 *  
 * @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");  
    }  
}       

測試類CloseResourceIn7.java

/**  
 * jdk7及以後關閉流的方式  
 *  
 * @author hetiantian  
 * */  
public class CloseResourceIn7 {  
    public static void main(String[] args) {  
        try(Resource resource = new Resource()) {  
            resource.sayHello();  
        } catch (Exception e) {  
            e.printStackTrace();  
        }  
    }  
}        

列印結果：

hello  
Resource is closed      

當存在多個打開資源的時候：資源二Resource2.java

/**  
 * 資源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");  
    }  
}        

/**  
 * jdk7及以後關閉流的方式  
 *  
 * @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();  
        }  
    }  
}        

`hello  
Resource say hello  
Resource2 is closed  
Resource is closed  
`      

即使資源很多，代碼也可以寫的很簡潔，如果用JDK7之前的方式去關閉資源，那麼資源越多，用fianl關閉資源時嵌套也就越多。最近寫的這篇：寫了個全局變量的bug，也推薦看下。

那麼它的底層原理又是怎樣的呢，由皮皮甜獨家揭秘優雅關閉資源背後的密碼秘密

檢視編譯的class檔案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();  
        }  

    }  
}        

可以發現編譯以後生成了try-catch-finally語句塊 finally中的var2.addSuppressed(var11);

是不是有疑問？其實這麼做是為了處理異常屏蔽的，我們将代碼修改一下。

資源Resource.java

/**  
 * 資源類  
 *  
 * @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");  
    }  
}        

兩個方法裡面都抛出異常

/**  
 * jdk7及以後關閉流的方式  
 *  
 * @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();  
            }  
        }  
    }  
}        

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)        

隻列印了最後出現的異常【異常屏蔽】這樣會給開發人員排查錯誤帶來一定的困難 我們換成try-with-resource方法實作CloseResourceIn7.java

/**  
 * jdk7及以後關閉流的方式  
 *  
 * @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();  
        }  

    }  
}        

列印資訊：

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      

可以發現，異常資訊中多了一個Suppressed的提示，告訴我們這個異常其實由兩個異常組成，Close method throw Exception這個異常是被Suppressed【屏蔽】的異常