Java如何正确关闭IO流

以你之姓@ 2021-09-25 15:42 373阅读 0赞

**一、错误示例1**

public void CopyFile() {
        FileReader fr = null;
        FileWriter fw = null;
        try {
            fr = new FileReader("c:\\xy1.txt"); // ①
            fw = new FileWriter("c:\\xy2.txt"); // ②
            char[] charBuffer = new char[1024];
            int len = 0;
            while ((len = fr.read(charBuffer)) != -1) {
                fw.write(charBuffer, 0, len);
            }
            System.out.println("文件复制成功");
        } catch (IOException e) {
            throw new RuntimeException("文件拷贝操作失败");
        } finally {
            try {
                fr.close(); // ③
                fw.close(); // ④
            } catch (IOException e) {
                throw new RuntimeException("关闭失败");
            }
        }
    }

如果①代码出错，fr根本就没有初始化，执行③的时候就会报空指针异常。②④同样是这个道理。  
  
  
**二、错误示例2**

public void CopyFile() {
        FileReader fr = null;
        FileWriter fw = null;
        try {
            fr = new FileReader("c:\\xy1.txt"); // ①
            fw = new FileWriter("c:\\xy2.txt"); // ②
            char[] charBuffer = new char[1024];
            ?
            int len = 0;
            while ((len = fr.read(charBuffer)) != -1) {
                fw.write(charBuffer, 0, len);
            }
            System.out.println("文件复制成功");
        } catch (IOException e) {
            throw new RuntimeException("文件拷贝操作失败");
        } finally {
            try {
                if (null != fr) {
                    fr.close(); // ③
                }
                if (null != fw) {
                    fw.close(); // ④
                }
            } catch (IOException e) {
                throw new RuntimeException("关闭失败"); // ⑤
            }
        }
    }

加上是否为空的判断可以避免空指针异常。但是如果③执行出错，程序会直接进入⑤而④根本没有得到执行，导致无法关闭。  
  
**三、正确示例**

public void CopyFile() {
        FileReader fr = null;
        FileWriter fw = null;
        try {
            fr = new FileReader("c:\\xy1.txt");
            fw = new FileWriter("c:\\xy2.txt");
            char[] charBuffer = new char[1024];
            ?
            int len = 0;
            while ((len = fr.read(charBuffer)) != -1) {
                fw.write(charBuffer, 0, len);
            }
            System.out.println("文件复制成功");
        } catch (IOException e) {
            throw new RuntimeException("文件拷贝操作失败");
        } finally {
            try {
                if (null != fr) {
                    fr.close();
                }
            } catch (IOException e) {
                throw new RuntimeException("关闭失败");
            }
    
            try {
                if (null != fw) {
                    fw.close();
                }
            } catch (IOException e) {
                throw new RuntimeException("关闭失败");
            }
        }
    }