从AssetManager读取文件作为InputStream后的java.lang.NullPointerException

Question

AssetManager定义和使用：

...
AssetManager mngr = getAssets();
try{
      encrypter.decrypt(mngr.open("sample.txt"),output_file);
}...(continued)

解密功能：

public void decrypt(InputStream in, OutputStream out)
    {
            try
            {
                    // Bytes read from in will be decrypted
                    in = new CipherInputStream(in, dcipher);
                    // Read in the decrypted bytes and write the cleartext to out
                    int numRead = 0;
                    while ((numRead = in.read(buf)) >= 0)
                    {
                      out.write(buf, 0, numRead);
                    }
                    out.close();
            ...(continued)

它给行 out.write（buf，0，numRead）; 作为java.lang.NullPointerException的错误。

此调用在以下情况下正常运行：

* encrypter.decrypt（new FileInputStream（“sample.txt”），output_file）*（即从本地读取文件而不是android的assets目录）;

有什么原因吗？

任何帮助表示赞赏！谢谢！

Answer 1

你的OutputStream没有初始化。所以你得到空指针异常。你可能想要 像这样的东西

 public void decrypt(InputStream in,String file)
        {
                try
                {
                 OutputStream out = new FileOutputStream(file);
                        // Bytes read from in will be decrypted
                        in = new CipherInputStream(in, dcipher);
                        // Read in the decrypted bytes and write the cleartext to out
                        int numRead = 0;
                        while ((numRead = in.read(buf)) >= 0)
                        {
                          out.write(buf, 0, numRead);
                        }
                        out.close();