Java – Try with Resources

1.介绍

Java 7中引入的对try-with-resources的支持使我们能够声明将在try块中使用的资源，并确保在执行该块后将关闭资源。

⚠️：声明的资源必须实现AutoCloseable接口。

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2.使用try-with-resources

简单地说，要自动关闭，必须在try中声明和初始化资源，如下所示：

try (PrintWriter writer = new PrintWriter(new File("test.txt"))) {
    writer.println("Hello World");
}

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3.使用try-with-resources代替try–catch-finally

使用简单明显的try-with-resources代替传统和冗长的try-catch-finally 块。

让我们比较以下代码示例–首先是一个典型的try-catch-finally块，然后是新方法，使用等效的try-with-resources块：

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();
    }
}

这是使用try-with-resources的超级简洁解决方案：

try (Scanner scanner = new Scanner(new File("test.txt"))) {
    while (scanner.hasNext()) {
        System.out.println(scanner.nextLine());
    }
} catch (FileNotFoundException fnfe) {
    fnfe.printStackTrace();
}

你最钟意那种呢？

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4.具有多个资源的try-with-resources

通过用分号分隔多个资源，可以在try-with-resources块中声明多个资源：

try (Scanner scanner = new Scanner(new File("testRead.txt"));
    PrintWriter writer = new PrintWriter(new File("testWrite.txt"))) {
    while (scanner.hasNext()) {
    writer.print(scanner.nextLine());
    }
}

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5.自定义实现AutoCloseable接口的资源

要构造一个将由try-with-resources块正确处理的自定义资源，该类应需要实现Closeable或AutoCloseable接口，并覆盖close方法：

public class MyResource implements AutoCloseable {
    @Override
    public void close() throws Exception {
        System.out.println("Closed MyResource");
    }
}

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6.资源关闭顺序

资源首先将会被定义（创建），然后使用，最后被关闭。让我们看一下示例：

资源1:

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");
    }
}

资源2:

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");
    }
}

使用资源：

private void orderOfClosingResources() throws Exception {
    try (AutoCloseableResourcesFirst af = new AutoCloseableResourcesFirst();
        AutoCloseableResourcesSecond as = new AutoCloseableResourcesSecond()) {

        af.doSomething();
        as.doSomething();
    }
}

输出：

Constructor -> AutoCloseableResources_First

Constructor -> AutoCloseableResources_Second

Something -> AutoCloseableResources_First

Something -> AutoCloseableResources_Second

Closed AutoCloseableResources_Second

Closed AutoCloseableResources_First

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7.catch&finally

一个带有资源的try块仍然可以包含catch和finally块，其工作方式与传统try块相同。

8.总结

在本文中，我们讨论了如何使用try-with-resources，如何用try-with-resources替换try，catch和finally，如何使用AutoCloseable构建自定义资源以及了解关闭资源的顺序。

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