java - finally { if (inputStream != null) { inputStream.close();

Question

我不知道怎么理解:

{
        if (inputStream **!= null**) {
            inputStream.close();

来自那个例子:

public class CopyLines {
public static void main(String[] args) throws IOException {

    BufferedReader inputStream = null;
    PrintWriter outputStream = null;

    try {
        inputStream = new BufferedReader(new FileReader("xanadu.txt"));
        outputStream = new PrintWriter(new FileWriter("characteroutput.txt"));

        String l;
        while ((l = inputStream.readLine()) != null) {
            outputStream.println(l);
        }
    } finally {
        if (inputStream != null) {
            inputStream.close();
        }
        if (outputStream != null) {
            outputStream.close();
        }
    }
}}

当提供任何数据时，inputStream 正在关闭？？？

Answer 1

这意味着无论何时 try block 完成(成功与否)，它都会尝试关闭 inputStream 中的流( outputStream 和 finally ) block ，但作为 try创建 BufferedReader 的实例时 block 可能会失败或 PrintWriter , 你需要先检查它是否不是 null否则你会得到一个 NPE。

可以考虑使用 try-with-resouces 语句以避免必须检查 null并调用 close()显式地这样可以大大简化您的代码。

try (BufferedReader inputStream = new BufferedReader(new FileReader("xanadu.txt"));
     PrintWriter outputStream = new PrintWriter(new FileWriter("characteroutput.txt")) {
    // your code here
}