关于JSP中判断文件目录下是否有文件或子目录的问题，在线求详细解答，本人新手。

list()返回由此抽象路径名所表示的目录中的文件和目录的名称所组成字符串数组。

如果此抽象路径名并不表示一个目录，则此方法将返回 null.否则，为目录中的每个文件或目录返回一个字符串数组。表示目录本身及其父目录的名称不包括在结果中。每个字符串是一个文件名，而不是一条完整路径。

SUN写这个方法的时候就规定了可以返回null，是防止路径不是目录。

而用files.length != 0，是在files不为null得条件下继续判断，表明此时目录下面至少有一个文件或者子目录存在。

JAVA里面&&判断是截断的，即就是如果files != null为假，就不会判断后面的了呢。

public static void copy(String from_name, String to_name)

throws IOException

{

File from_file = new File(from_name) // Get File objects from Strings

File to_file = new File(to_name)

String parent = to_file.getParent() // The destination directory

if (parent == null) // If none, use the current directory

parent = System.getProperty("user.dir")

File dir = new File(parent) // Convert it to a file.

dir.mkdir()

// If we've gotten this far, then everything is okay.

// So we copy the file, a buffer of bytes at a time.

FileInputStream from = null // Stream to read from source

FileOutputStream to = null // Stream to write to destination

try {

from = new FileInputStream(from_file) // Create input stream

to = new FileOutputStream(to_file)// Create output stream

byte[] buffer = new byte[4096]// To hold file contents

int bytes_read// How many bytes in buffer

// Read a chunk of bytes into the buffer, then write them out,

// looping until we reach the end of the file (when read() returns

// -1). Note the combination of assignment and comparison in this

// while loop. This is a common I/O programming idiom.

while((bytes_read = from.read(buffer)) != -1) // Read until EOF

to.write(buffer, 0, bytes_read) // write

}

finally {

if (from != null) try { from.close()} catch (IOException e) { }

if (to != null) try { to.close()} catch (IOException e) { }

}

}

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