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。
:warning:：声明的资源必须实现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，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？
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());
}
}
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");
}
}
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
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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