使用try-with-resources優雅關閉資源

代碼必定得寫的優雅一點

你還在使用try-catch-finally關閉資源嗎，若是是，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以前資源的關閉姿式：

/**
 * jdk7之前關閉流的方式
 *
 * @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();
            }
        }
    }
}
複製代碼

JDK7及之後關閉資源的正確姿式：try-with-resource Resource的定義：全部實現了 java.lang.AutoCloseable 接口（其中，它包括實現了 java.io.Closeable 的全部對象），能夠使用做爲資源。 簡單Demo進行證明： 實現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");
    }
}
複製代碼

測試類CloseResourceIn7.java

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

打印結果：

hello
Resource is closed
複製代碼

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");
    }
}
複製代碼

測試類CloseResourceIn7.java

/**
 * jdk7及之後關閉流的方式
 *
 * @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();
        }
    }
}
複製代碼

打印結果：

hello
Resource say hello
Resource2 is closed
Resource is closed
複製代碼

即便資源不少，代碼也能夠寫的很簡潔，若是用jdk7以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

那麼它的底層原理又是怎樣的呢，由皮皮甜獨家揭祕優雅關閉資源背後的密碼 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();
        }

    }
}
複製代碼

能夠發現編譯之後生成了try-catch-finally語句塊 finally中的var2.addSuppressed(var11);是否是有疑問🤔️ 其實這麼作是爲了處理異常屏蔽的 咱們將代碼修改一下 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");
    }
}
複製代碼

兩個方法裏面都拋出異常

測試類CloseResourceIn7.java

/**
 * jdk7及之後關閉流的方式
 *
 * @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();
            }
        }
    }
}
複製代碼

打印結果：

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)
複製代碼

只打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必定的困難 咱們換成try-with-resource方法實現CloseResourceIn7.java

/**
 * jdk7及之後關閉流的方式
 *
 * @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();
        }

    }
}
複製代碼

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
複製代碼

能夠發現，異常信息中多了一個Suppressed的提示，告訴咱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

總結 怎麼樣，是否是很簡單呢，若是學會了話