空文件判断方法分析

本文结合File,FileInputStream,FileRedaer以及BufferedReader的方法，将给出四个判断文件是否为空的方法：




本文包括如下三个部分：
1. 贴出File,FileInputStream,FileRedaer以及BufferedReader相应方法的源代码，了解一下。
2. 给出自己的实现。
3. 测试并给出分析结论。

File,FileInputStream,FileRedaer以及BufferedReader相应方法的源代码

/**
     * Returns the length of the file denoted by this abstract pathname.
     * The return value is unspecified if this pathname denotes a directory.
     *
     * @return  The length, in bytes, of the file denoted by this abstract
     *          pathname, or <code>0L</code> if the file does not exist
     *
     * @throws  SecurityException
     *          If a security manager exists and its <code>{@link
     *          java.lang.SecurityManager#checkRead(java.lang.String)}</code>
     *          method denies read access to the file
     */
    public long length() {
	SecurityManager security = System.getSecurityManager();
	if (security != null) {
	    security.checkRead(path);
	}
	return fs.getLength(this);
    }

public
class FileInputStream extends InputStream
{
    /**
     * Returns the number of bytes that can be read from this file input
     * stream without blocking.
     *
     * @return     the number of bytes that can be read from this file input
     *             stream without blocking.
     * @exception  IOException  if an I/O error occurs.
     */
public native int available() throws IOException;
}

public class InputStreamReader extends Reader {
/**
     * Tell whether this stream is ready to be read.  An InputStreamReader is
     * ready if its input buffer is not empty, or if bytes are available to be
     * read from the underlying byte stream.
     *
     * @exception  IOException  If an I/O error occurs
     */
    public boolean ready() throws IOException {
	return sd.ready();
    }
}

public class FileReader extends InputStreamReader {

}

public class BufferedReader extends Reader {

/**
     * Tell whether this stream is ready to be read.  A buffered character
     * stream is ready if the buffer is not empty, or if the underlying
     * character stream is ready.
     *
     * @exception  IOException  If an I/O error occurs
     */
    public boolean ready() throws IOException {
	synchronized (lock) {
	    ensureOpen();

	    /* 
	     * If newline needs to be skipped and the next char to be read
	     * is a newline character, then just skip it right away.
	     */
	    if (skipLF) {
		/* Note that in.ready() will return true if and only if the next 
		 * read on the stream will not block.
		 */
		if (nextChar >= nChars && in.ready()) {
		    fill();
		}
		if (nextChar < nChars) {
		    if (cb[nextChar] == '\n') 
			nextChar++;
		    skipLF = false;
		} 
	    }
	    return (nextChar < nChars) || in.ready();
	}
}
}

给出自己的实现类

package my.tool.file.util;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;

public class EmptyFileChecker {

	public static boolean isFileEmpty1(File file) {
		FileInputStream fis = null;
		boolean flag = true;
		try {
			fis = new FileInputStream(file);
			if (fis.available() != 0) {
				flag = false;
			}
		} catch (FileNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} finally {
			if (null != fis) {
				try {
					fis.close();
				} catch (IOException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				} finally {
					fis = null;
				}
			}
		}

		return flag;
	}

	public static boolean isFileEmpty2(File file) {
		boolean flag = true;
		FileReader fr = null;
		try {
			fr = new FileReader(file);

			if (fr.ready()) {
				flag = false;
			}
		} catch (FileNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} finally {
			if (null != fr) {
				try {
					fr.close();
				} catch (IOException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				} finally {
					fr = null;
				}
			}
		}
		return flag;
	}

	public static boolean isFileEmpty3(File file) {
		boolean flag = true;

		BufferedReader br = null;

		try {
			br = new BufferedReader(new FileReader(file));

			if (br.ready()) {
				flag = false;
			}
		} catch (FileNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} finally {
			if (null != br) {
				try {
					br.close();
				} catch (IOException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				} finally {
					br = null;
				}
			}
		}

		return flag;
	}
	
	public static boolean isFileEmpty4(File file) {
		return file.length() ==0L;
	}
 }

为了测试文件是否为空，新建多种文件类型的文件。



Note: 新建文件后，像zip文件它们的大小都不是0KB。

编写一个测试类：

package my.tool.file.util;

import java.io.File;

public class Main {
	public static void main(String[] args) {
		for(File file: new File("D: \\EmptyFileFolder").listFiles())
		{
			System.out.print(file.getName());
			System.out.print(" ");
			System.out.print(" is empty ? ");
			System.out.println();
			
			System.out.println("Call method available in FileInputStream -->" + EmptyFileChecker.isFileEmpty1(file));
			
			System.out.println("Call method ready in FileReader -->" + EmptyFileChecker.isFileEmpty2(file));
			
			System.out.println("Call method ready in BufferedReader -->" + EmptyFileChecker.isFileEmpty3(file));
			
			System.out.println("Call method length in File -->" + EmptyFileChecker.isFileEmpty4(file));
			
			System.out.println();
		}
	}
	
}

输出的结果如下：

新建 BMP 图像.bmp  is empty ?
Call method available in FileInputStream -->true
Call method ready in FileReader -->true
Call method ready in BufferedReader -->true
Call method length in File -->true

新建 Microsoft Office Access 2007 Database.accdb  is empty ?
Call method available in FileInputStream -->false
Call method ready in FileReader -->false
Call method ready in BufferedReader -->false
Call method length in File -->false

新建 Microsoft Office Word Document.docx  is empty ?
Call method available in FileInputStream -->true
Call method ready in FileReader -->true
Call method ready in BufferedReader -->true
Call method length in File -->true

新建 OpenDocument Drawing.odg  is empty ?
Call method available in FileInputStream -->false
Call method ready in FileReader -->false
Call method ready in BufferedReader -->false
Call method length in File -->false

新建 OpenDocument Spreadsheet.ods  is empty ?
Call method available in FileInputStream -->false
Call method ready in FileReader -->false
Call method ready in BufferedReader -->false
Call method length in File -->false

新建 压缩(zipped)文件夹.zip  is empty ?
Call method available in FileInputStream -->false
Call method ready in FileReader -->false
Call method ready in BufferedReader -->false
Call method length in File -->false

新建 文本文档.txt  is empty ?
Call method available in FileInputStream -->true
Call method ready in FileReader -->true
Call method ready in BufferedReader -->true
Call method length in File -->true

新建 波形声音.wav  is empty ?
Call method available in FileInputStream -->false
Call method ready in FileReader -->false
Call method ready in BufferedReader -->false
Call method length in File -->false




结论： 通过File的length方法,FileInputStream的available方法或者是FileReader以及BufferedReader的ready方法，只有当新建的文件是0KB的时候，才能正确判断文件是否为空。如果新建后的文件有一定的大小，比如zip文件有1KB，这样的文件，如果想要知道文件里面有没有内容，只能采用其它方式来做。