你還在使用 try-catch-finally 關(guān)閉資源？不太優(yōu)雅~

代碼一定得寫的優(yōu)雅一點(diǎn)！

你還在使用try-catch-finally關(guān)閉資源嗎，如果是，那么就有點(diǎn)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(guān)閉姿勢(shì)：

/**
 * jdk7以前關(guān)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及以后關(guān)閉資源的正確姿勢(shì)

try-with-resource Resource的定義：

所有實(shí)現(xiàn)了 java.lang.AutoCloseable^[1] 接口（其中，它包括實(shí)現(xiàn)了 java.io.Closeable^[2] 的所有對(duì)象），可以使用作為資源。簡單Demo進(jìn)行證實(shí)：實(shí)現(xiàn)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(cè)試類CloseResourceIn7.java

/**
 * jdk7及以后關(guān)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();
        }
    }
}

打印結(jié)果：

hello
Resource is closed

當(dāng)存在多個(gè)打開資源的時(shí)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");
    }
}

測(cè)試類CloseResourceIn7.java

/**
 * jdk7及以后關(guān)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();
        }
    }
}

打印結(jié)果：

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(guān)閉資源，那么資源越多，用fianl關(guān)閉資源時(shí)嵌套也就越多。

那么它的底層原理又是怎樣的呢，由皮皮甜獨(dú)家揭秘優(yōu)雅關(guān)閉資源背后的密碼秘密

查看編譯的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();
        }

    }
}

可以發(fā)現(xiàn)編譯以后生成了try-catch-finally語句塊 finally中的var2.addSuppressed(var11);

是不是有疑問？其實(shí)這么做是為了處理異常屏蔽的，我們將代碼修改一下。

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(gè)方法里面都拋出異常

測(cè)試類CloseResourceIn7.java

/**
 * jdk7及以后關(guān)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();
            }
        }
    }
}

打印結(jié)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)

只打印了最后出現(xiàn)的異?！井惓Ｆ帘巍窟@樣會(huì)給開發(fā)人員排查錯(cuò)誤帶來一定的困難 我們換成try-with-resource方法實(shí)現(xiàn)CloseResourceIn7.java

/**
 * jdk7及以后關(guān)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();
        }

    }
}

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

可以發(fā)現(xiàn)，異常信息中多了一個(gè)Suppressed的提示，告訴我們這個(gè)異常其實(shí)由兩個(gè)異常組成，Close method throw Exception這個(gè)異常是被Suppressed【屏蔽】的異常

參考資料