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);    }

publicclass 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 blocke.printStackTrace();} catch (IOException e) {// TODO Auto-generated catch blocke.printStackTrace();} finally {if (null != fis) {try {fis.close();} catch (IOException e) {// TODO Auto-generated catch blocke.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 blocke.printStackTrace();} catch (IOException e) {// TODO Auto-generated catch blocke.printStackTrace();} finally {if (null != fr) {try {fr.close();} catch (IOException e) {// TODO Auto-generated catch blocke.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 blocke.printStackTrace();} catch (IOException e) {// TODO Auto-generated catch blocke.printStackTrace();} finally {if (null != br) {try {br.close();} catch (IOException e) {// TODO Auto-generated catch blocke.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。