说说Java7 之 Try with Resources

参考链接： Java try-with-resources

说说Java7 之 Try with Resources 

java7引入Try with Resources语法，允许我们在try块中声明并使用资源，确保在使用之后资源被关闭。资源必须实现AuthCloseable接口。 

使用Try with Resources 

简单地说，为了自动关闭，资源必须在try块中声明并初始化，示例如下： 

try (PrintWriter writer = new PrintWriter(new File("test.txt"))) {

    writer.println("Hello World");

} 

使用Try with Resources 代替 try–catch-finally 

上面方式非常简洁，因此建议使用Try with Resources 代替传统的冗长方式：try–catch-finally。  下面通过示例比较两种方式，首先是采用try–catch-finally，然后采用新的方法，通过Try with Resources实现等价功能： 

Scanner scanner = null;

try {

    scanner = new Scanner(new File("test.txt"));

    while (scanner.hasNext()) {

        System.out.println(scanner.nextLine());

    }

} catch (FileNotFoundException e) {

    e.printStackTrace();

} finally {

    if (scanner != null) {

        scanner.close();

    }

} 

然后，使用Try with Resources实现超级简洁的解决方案： 

try (Scanner scanner = new Scanner(new File("test.txt"))) {

    while (scanner.hasNext()) {

        System.out.println(scanner.nextLine());

    }

} catch (FileNotFoundException fnfe) {

    fnfe.printStackTrace();

} 

try-with-resources 与多个资源 

可以在try-with-resources 块中声明多个资源，通过分号分割： 

try (Scanner scanner = new Scanner(new File("testRead.txt"));

    PrintWriter writer = new PrintWriter(new File("testWrite.txt"))) {

    while (scanner.hasNext()) {

    writer.print(scanner.nextLine());

    }

} 

使用AutoCloseable接口自定义资源 

为了创建自定义可被 try-with-resources块自动处理的资源，则该类需要实现Closeable 或 AutoCloseable 接口，然后重载其close方法： 

public class MyResource implements AutoCloseable {

    @Override

    public void close() throws Exception {

        System.out.println("Closed MyResource");

    }

} 

关闭资源顺序 

最先定义的资源最后被关闭，请看示例： 

第一个资源： 

public class AutoCloseableResourcesFirst implements AutoCloseable {

    public AutoCloseableResourcesFirst() {

        System.out.println("Constructor -> AutoCloseableResources_First");

    }

    public void doSomething() {

        System.out.println("Something -> AutoCloseableResources_First");

    }

    @Override

    public void close() throws Exception {

        System.out.println("Closed AutoCloseableResources_First");

    }

} 

第二个资源： 

public class AutoCloseableResourcesSecond implements AutoCloseable {

    public AutoCloseableResourcesSecond() {

        System.out.println("Constructor -> AutoCloseableResources_Second");

    }

    public void doSomething() {

        System.out.println("Something -> AutoCloseableResources_Second");

    }

    @Override

    public void close() throws Exception {

        System.out.println("Closed AutoCloseableResources_Second");

    }

} 

调用代码： 

private void orderOfClosingResources() throws Exception {

    try (AutoCloseableResourcesFirst af = new AutoCloseableResourcesFirst();

        AutoCloseableResourcesSecond as = new AutoCloseableResourcesSecond()) {

        af.doSomething();

        as.doSomething();

    }

} 

输出结果： 

Constructor -> AutoCloseableResources_First

Constructor -> AutoCloseableResources_Second

Something -> AutoCloseableResources_First

Something -> AutoCloseableResources_Second

Closed AutoCloseableResources_Second

Closed AutoCloseableResources_First 

catch 和 finally 

使用try-with-resources块，仍可以有catch 和 finally，它们功能和传统的try块一样。 

总结 

本文我们讨论了如何使用try-with-resources，通过示例说明如何使用try-with-resources代替try, catch 和 finally。使用AutoCloseable 接口自定义资源以及资源关闭顺序。