說說Java7 之 Try with Resources

java7引入Try with Resources文法，允許我們在try塊中聲明并使用資源，確定在使用之後資源被關閉。資源必須實作AuthCloseable接口。

使用Try with Resources

簡單地說，為了自動關閉，資源必須在try塊中聲明并初始化，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

上面方式非常簡潔，是以建議使用Try with Resources 代替傳統的冗長方式：try–catch-finally。

下面通過示例比較兩種方式，首先是采用try–catch-finally，然後采用新的方法，通過Try with Resources實作等價功能：

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();
    }
}
           

然後，使用Try with Resources實作超級簡潔的解決方案：

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 與多個資源

可以在try-with-resources 塊中聲明多個資源，通過分号分割：

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接口自定義資源

為了建立自定義可被 try-with-resources塊自動處理的資源，則該類需要實作Closeable 或 AutoCloseable 接口，然後重載其close方法：

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

關閉資源順序

最先定義的資源最後被關閉，請看示例：

第一個資源：

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");
    }
}
           

第二個資源：

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");
    }
}
           

調用代碼：

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

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

輸出結果：

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塊，仍可以有catch 和 finally，它們功能和傳統的try塊一樣。

總結

本文我們讨論了如何使用try-with-resources，通過示例說明如何使用try-with-resources代替try, catch 和 finally。使用AutoCloseable 接口自定義資源以及資源關閉順序。