try-with-resource 自动关闭 AutoClosable 接口的资源

JAVA 中，JVM 的垃圾回收机制可以对内部资源实现自动回收，给开发者带来了极大的便利。但是 JVM 对外部资源(调用了底层操作系统的资源)的引用却无法自动回收，例如数据库连接，网络连接以及输入输出 IO 流等。这些连接就需要我们手动去关闭，不然会导致外部资源泄露，连接池溢出以及文件被异常占用等。JDK7 之后，新增了“ try-with-resource”。它可以自动关闭实现了AutoClosable 接口的类，实现类需要实现 close()方法。”try-with-resources 声明”，将 try-catch-finally 简化为 try-catch，这其实是一种语法糖，在编译时仍然会进行转化为 try-catch-finally 语句。

常见的实现了AutoCloseable接口的类有:

BufferedInputStream, BufferedOutputStream

BufferedReader, BufferedWriter

FileInputStream, FileOutputStream

FileReader, FileWriter

InputStream, OutputStream

PrintWriter, PrintStream

Reader, Writer

Scanner, SSLServerSocker, SSLSocket等等等等

举例说明

try–catch-finally 实现

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();
    }
}

try-with-resources 实现

try (Scanner scanner = new Scanner(new File("testRead.txt")); 
PrintWriter writer = new PrintWriter(new File("testWrite.txt"))) {

    while (scanner.hasNext()) {
        writer.print(scanner.nextLine());
    }

}catch (FileNotFoundException e) {
    e.printStackTrace();
}