1. 概述
对try-with-resources的支持在Java 7中引入,允许我们声明要在try块中使用的资源,并确保资源将在该块执行后关闭。
声明的资源需要实现AutoCloseable接口。
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:
try (PrintWriter writer = new PrintWriter(new File("test.txt"))) {
writer.println("Hello World");
}
3. 将try-catch-finally替换为try-with-resources
使用新的try-with-resources功能的简单而明显的方法是替换传统且冗长的try-catch-finally块。
让我们比较以下代码示例。
第一个是典型的try-catch-finally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的新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();
}
4. try-with-resources多个资源
我们可以在try-with-resources块中通过用分号分隔它们来声明多个资源:
try (Scanner scanner = new Scanner(new File("testRead.txt"));
PrintWriter writer = new PrintWriter(new File("testWrite.txt"))) {
while (scanner.hasNext()) {
writer.print(scanner.nextLine());
}
}
5. 具有AutoCloseable的自定义资源
要构造将由try-with-resources块正确处理的自定义资源,该类应实现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");
}
}
6. 资源关闭顺序
首先定义/获取的资源将最后关闭。让我们看一下此行为的示例:
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");
}
}
代码:
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
7. catch和finally
try-with-resources块仍然可以有catch和finally块,它们的工作方式与传统的try块相同。
8.Java 9-有效最终变量
在Java 9之前,我们只能在try-with-resources块中使用新变量:
try (Scanner scanner = new Scanner(new File("testRead.txt"));
PrintWriter writer = new PrintWriter(new File("testWrite.txt"))) {
// omitted
}
如上所示,这在声明多个资源时尤其冗长。从Java 9开始,作为JEP 213的一部分,我们现在可以在try-with-resources块中使用final甚至有效final变量:
final Scanner scanner = new Scanner(new File("testRead.txt"));
PrintWriter writer = new PrintWriter(new File("testWrite.txt"))
try (scanner;writer) {
// omitted
}
简而言之,如果一个变量在第一次赋值后没有改变,那么它实际上是最终的,即使它没有明确标记为final。
如上所示,scanner变量被显式声明为final,因此我们可以将其与try-with-resources块一起使用。而writer变量虽然不是明确的final,但它在第一次赋值后不会改变。因此,我们也可以使用writer变量。
9. 总结
在本文中,我们讨论了如何使用try-with-resources以及如何用try-with-resources替换try、catch和finally。
我们还研究了使用AutoCloseable构建自定义资源以及资源关闭的顺序。
与往常一样,本教程的完整源代码可在GitHub上获得。