1.介紹
Java 7中引入的對try-with-resources的支持使我們能夠聲明將在try塊中使用的資源，並確保在執行該塊后將關閉資源。
⚠️：聲明的資源必須實現AutoCloseable接口。
2.使用try-with-resources
簡單地說，要自動關閉，必須在try中聲明和初始化資源，如下所示：
try (PrintWriter writer = new PrintWriter(new File("test.txt"))) {
writer.println("Hello World");
}
3.使用try-with-resources代替try–catch-finally
使用簡單明顯的try-with-resources代替傳統和冗長的try-catch-finally 塊。
讓我們比較以下代碼示例–首先是一個典型的try-catch-finally塊，然后是新方法，使用等效的try-with-resources塊：
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();
}
}
這是使用try-with-resources的超級簡潔解決方案：
try (Scanner scanner = new Scanner(new File("test.txt"))) {
while (scanner.hasNext()) {
System.out.println(scanner.nextLine());
}
} catch (FileNotFoundException fnfe) {
fnfe.printStackTrace();
}
你最鍾意那種呢？
4.具有多個資源的try-with-resources
通過用分號分隔多個資源，可以在try-with-resources塊中聲明多個資源：
try (Scanner scanner = new Scanner(new File("testRead.txt"));
PrintWriter writer = new PrintWriter(new File("testWrite.txt"))) {
while (scanner.hasNext()) {
writer.print(scanner.nextLine());
}
}
5.自定義實現AutoCloseable接口的資源
要構造一個將由try-with-resources塊正確處理的自定義資源，該類應需要實現Closeable或AutoCloseable接口，並覆蓋close方法：
public class MyResource implements AutoCloseable {
@Override
public void close() throws Exception {
System.out.println("Closed MyResource");
}
}
6.資源關閉順序
資源首先將會被定義（創建），然后使用，最后被關閉。讓我們看一下示例：
資源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");
}
}
資源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");
}
}
使用資源：
private void orderOfClosingResources() throws Exception {
try (AutoCloseableResourcesFirst af = new AutoCloseableResourcesFirst();
AutoCloseableResourcesSecond as = new AutoCloseableResourcesSecond()) {
af.doSomething();
as.doSomething();
}
}
輸出：
Constructor -> AutoCloseableResources_First
Constructor -> AutoCloseableResources_Second
Something -> AutoCloseableResources_First
Something -> AutoCloseableResources_Second
Closed AutoCloseableResources_Second
Closed AutoCloseableResources_First
7.catch&finally
一個帶有資源的try塊仍然可以包含catch和finally塊，其工作方式與傳統try塊相同。
8.總結
在本文中，我們討論了如何使用try-with-resources，如何用try-with-resources替換try，catch和finally，如何使用AutoCloseable構建自定義資源以及了解關閉資源的順序。
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