Question

我正在试图确保将url中的内容成功写入文件。为此，我使用以下代码

public void copyFileFromUrl(URL source, File target, int count) throws IOException {

    InputStream in = null;
    OutputStream out = null;

    if (target != null) {
        try {       
            if (!target.exists()) {
                target.createNewFile();
                log.debug("target file created for " + target);
                log.debug("downloading source .... " + source);

                if (source == null) {                   
                    log.debug("Null source .... " + ScormFileWriter.class.getName());
                    return;         
                } else {    
                    in = source.openStream();   
                }   
                out = new FileOutputStream(target);

                byte[] buf = new byte[1024];
                int len;
                while ((len = in.read(buf)) > 0) {
                    out.write(buf, 0, len);
                }

                log.debug("The contents from the URL: " + source + " are successfully written to the file " + target);

            } else {
                log.debug("skipping creation of asset");
            }

        } catch (Exception e) {

            if(count < 3){
                log.debug("trouble with " + target);
                if (in != null) {       
                    in.close(); 
                }
                if (out != null) {  
                     out.close();   
                }

                // Attempt to delete it
                boolean success = target.delete();

                if (!success) { 
                    log.debug("Unable to delete " + target);    
                } else {
                    copyFileFromUrl(source, target, ++count);   
                }               
            }

            log.debug("trouble in downloading source after trying " + count +  " times: " + source);
            e.printStackTrace();        
        } finally {         
            if (in != null) {               
                in.close();             
            }           
            if (out != null) {              
                 out.close();               
            }             
        }       
    }   
}

现在，当函数进入

时首次调用时会发生什么

while ((len = in.read(buf)) > 0) {
    out.write(buf, 0, len);
}

我拔掉了我的电缆，抛出了异常并且代码来阻塞并再次调用该函数。现在我插上电缆，这次while循环完成，行

log.debug("The contents from the URL: " + source + " are successfully written to the file " + target);

打印，代码到达最后阻止，然后代码到达这两行

log.debug("trouble in downloading source after trying " + count +  " times: " + source);
e.printStackTrace();

为什么呢？这次没有抛出任何异常，一切正常，为什么代码来阻塞？这次最终代码应该恢复正常吗？

由于

Answer 1

您递归调用该方法。第一次抛出异常时，代码将再次调用自身，直到执行线程从第二次调用方法返回时才会到达打印行。一旦方法正确完成，执行将返回到第一个＆＃34;实例＆＃34;该方法和执行的方法落到印刷线上。我认为更好的方法是循环尝试获取文件而不是递归调用相同的方法。如果你必须递归地调用它，请确保方法在重新调用之前执行它所需的所有操作，执行方式。

修改
您总是可以将打印行移动到递归调用之前，这样当执行回到它时，该方法将无需执行任何操作，除非＆＃34; unroll＆＃34;递归调用。如果你想避免递归调用，我正在考虑让循环继续最多3次，但如果成功你退出循环，否则，让循环返回顶部。类似的东西：

InputStream in = null; OutputStream out = null; if (target != null) { while(n<3){ try { if (!target.exists()) { target.createNewFile(); log.debug("target file created for " + target); log.debug("downloading source .... " + source); if (source == null) { log.debug("Null source .... " + ScormFileWriter.class.getName()); return; } else { in = source.openStream(); } out = new FileOutputStream(target); byte[] buf = new byte[1024]; int len; while ((len = in.read(buf)) > 0) { out.write(buf, 0, len); } log.debug("The contents from the URL: " + source + " are successfully written to the file " + target); } else { log.debug("skipping creation of asset"); } n=4; or break; } catch (Exception e) { log.debug("trouble with " + target); if (in != null) { in.close(); } if (out != null) { out.close(); } // Attempt to delete it boolean success = target.delete(); if (!success) { log.debug("Unable to delete " + target); } else { // copyFileFromUrl(source, target, ++count); } n++; if(n == 2){ log.debug("trouble in downloading source after trying " + count + " times: " + source); e.printStackTrace(); } } finally { if (in != null) { in.close(); } if (out != null) { out.close(); } } }

当然，您可能需要调整日志记录和退出条件以满足您的特定需求

即使没有抛出异常，代码也会进入catch块

1 个答案: