Java 读取文件，返回文件内容为空

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请看下面两个程序，我是想读取文件以后，返回文件里面的内容，但是尝试了几次，不知道为什么第一个程序总是返回null。使用debug调试的时候发现在执行到while ((s1 = bufRead.readLine())!= null) {这句时s1 是有值的，但是到了finally语句块，s1 = null; 第二个程序是正确的。为什么错误A和B的s1都是null呢？请各位亲帮忙解答一下。

错误A====>>

public String readFile(String fileUrl){String s1 = "";int line = 1;BufferedReader bufRead  = null;try { File file  = new File(fileUrl);if (file.exists() && file.isFile() ) {FileInputStream fileIn = new FileInputStream(file);bufRead = new BufferedReader(new InputStreamReader(fileIn));while ((s1 = bufRead.readLine())!= null) {line++;}}} catch (FileNotFoundException e) {e.printStackTrace();} catch (IOException e) {e.printStackTrace();} finally{if (bufRead != null) {try {bufRead.close();} catch (IOException e) {e.printStackTrace();}}}return s1;}

错误B====>>

public String readFile(String fileUrl){String s1 = "";int line = 1;BufferedReader bufRead  = null;try { File file  = new File(fileUrl);if (file.exists() && file.isFile() ) {FileInputStream fileIn = new FileInputStream(file);bufRead = new BufferedReader(new InputStreamReader(fileIn));while ((s1 = bufRead.readLine())!= null) {    s1 = bufRead.readLine();                                    line++;}}} catch (FileNotFoundException e) {e.printStackTrace();} catch (IOException e) {e.printStackTrace();} finally{if (bufRead != null) {try {bufRead.close();} catch (IOException e) {e.printStackTrace();}}}return s1;}

正确===>>

public String readFile(String fileUrl){ StringBuffer str = new StringBuffer();String s1 = "";int line = 1;BufferedReader bufRead  = null;try { File file  = new File(fileUrl);if (file.exists() && file.isFile() ) {FileInputStream fileIn = new FileInputStream(file);bufRead = new BufferedReader(new InputStreamReader(fileIn));while (( s1 = bufRead.readLine())!= null) {str.append(s1);line++;} }} catch (FileNotFoundException e) {e.printStackTrace();} catch (IOException e) {e.printStackTrace();} finally{if (bufRead != null) {try {bufRead.close();} catch (IOException e) {e.printStackTrace();}}}return str.toString();}