为什么我们需要 FileInputStream in = null; try块之前的行？

Question

此代码来自 oracle i/o 教程：

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

    FileInputStream in = null;
    FileOutputStream out = null;

    try {
        in = new FileInputStream("xanadu.txt");
        out = new FileOutputStream("outagain.txt");
        int c;

为什么这些行

FileInputStream in = null;
FileOutputStream out = null;

不包括以这种方式尝试阻止（没有= null）？

 FileInputStream in = new FileInputStream("xanadu.txt");
 FileOutputStream out = new FileOutputStream("outagain.txt");

Answer 1

您需要在 try {...} 之外声明 in 和 out，因为您需要在 finally {...} 块中关闭这些资源。

FileInputStream in = null;
FileOutputStream out = null;

try {
  in = new FileInputStream("xanadu.txt");
  out = new FileOutputStream("outagain.txt");
  int c;
  ....
} catch(Exception e) {

} finally {
   try {
     if(in != null) {       
       in.close();
     }
     if(out != null) {
       out.close();
     }
   catch(Exception e) {...}
}

如果您在 try {...} 范围内声明它们，编译器会抱怨它们无法解析。

如果你这样做：

try {
  FileInputStream in = new FileInputStream("xanadu.txt");
  FileOutputStream out = new FileOutputStream("outagain.txt");
  int c;
  ....
} catch(Exception e) {

} finally {
   try {
     if(in != null) { //in could not be resolved error by compiler       
       in.close();
     }
     if(out != null) { //out could not be resolved...
       out.close();
     }
   catch(Exception e) {...}
}

Answer 2

因为 Inputstream 是一个沉重的资源。
您已经打开了一个 FileInputStream，现在在使用它时发生了一些异常。然后该流将保持打开状态，浪费资源。

所以你在 try 块之前用 null 初始化它，这样你就可以在 finally 块中关闭它，即清理资源的正确方法。

FileInputStream in=null
    try{
       }catch(IOException ioexcep){
    }
    finally {
    in.close;
    }

Answer 3

如果你在 try 块中声明和初始化流，在 finally 语句中如果你想尝试关闭流，编译器不知道 in[=15= 的值是什么] 和 out 以关闭流。

FileInputStream in = null; FileOutputStream out = null; try { in = new FileInputStream("xanadu.txt"); out = new FileOutputStream("outagain.txt"); } catch(Exception e) { ---- } finally { if(in!=null) { in.close(); } if(out!= null) { out.close(); } }

Answer 4

如果您使用的是 Java 7 或更高版本，您可以使用 try-with-resources，它将为您处理关闭。

 try(
      FileInputStream in = new FileInputStream("xanadu.txt");
      FileOutputStream out = new FileOutputStream("outagain.txt");
  ) {
    .... do stuff
  } catch (Exception e) {
    ... deal with the exception
  };

这是因为 FileInputStream 实现了 java.lang.AutoCloseable，所以它会在 try 块完成或抛出异常时关闭（）。

为什么我们需要 FileInputStream in = null; try块之前的行？

Why do we need FileInputStream in = null; line before try block?

java

oop

core

object

stream