The Commons IO MagicNumberFileFilter.java source code
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.commons.io.filefilter;
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.io.Serializable;
import java.util.Arrays;
import org.apache.commons.io.IOUtils;
/**
* <p>
* File filter for matching files containing a "magic number". A magic number
* is a unique series of bytes common to all files of a specific file format.
* For instance, all Java class files begin with the bytes
* <code>0xCAFEBABE.
* </p>
*
* <code> * File dir = new File(".");
* MagicNumberFileFilter javaClassFileFilter =
* MagicNumberFileFilter(new byte[] {(byte) 0xCA, (byte) 0xFE,
* (byte) 0xBA, (byte) 0xBE});
* String[] javaClassFiles = dir.list(javaClassFileFilter);
* for (String javaClassFile : javaClassFiles) {
* System.out.println(javaClassFile);
* }
* </pre>
*
* <p>
* Sometimes, such as in the case of TAR files, the
* magic number will be offset by a certain number of bytes in the file. In the
* case of TAR archive files, this offset is 257 bytes.
* </p>
*
* <code> * File dir = new File(".");
* MagicNumberFileFilter tarFileFilter =
* MagicNumberFileFilter("ustar", 257);
* String[] tarFiles = dir.list(tarFileFilter);
* for (String tarFile : tarFiles) {
* System.out.println(tarFile);
* }
* </pre>
* @since Commons IO 2.0
* @see FileFilterUtils#magicNumberFileFilter(byte[])
* @see FileFilterUtils#magicNumberFileFilter(String)
* @see FileFilterUtils#magicNumberFileFilter(byte[], long)
* @see FileFilterUtils#magicNumberFileFilter(String, long)
*/
public class MagicNumberFileFilter extends AbstractFileFilter implements
Serializable {
/**
* The serialization version unique identifier.
*/
private static final long serialVersionUID = -547733176983104172L;
/**
* The magic number to compare against the file's bytes at the provided
* offset.
*/
private final byte[] magicNumbers;
/**
* The offset (in bytes) within the files that the magic number's bytes
* should appear.
*/
private final long byteOffset;
/**
* <p>
* Constructs a new MagicNumberFileFilter and associates it with the magic
* number to test for in files. This constructor assumes a starting offset
* of <code>0.
* </p>
*
* <p>
* It is important to note that <em>the array is not cloned and that
* any changes to the magic number array after construction will affect the
* behavior of this file filter.
* </p>
*
* <code> * MagicNumberFileFilter javaClassFileFilter =
* MagicNumberFileFilter(new byte[] {(byte) 0xCA, (byte) 0xFE,
* (byte) 0xBA, (byte) 0xBE});
* </pre>
*
* @param magicNumber the magic number to look for in the file.
*
* @throws IllegalArgumentException if <code>magicNumber is
* <code>null, or contains no bytes.
*/
public MagicNumberFileFilter(byte[] magicNumber) {
this(magicNumber, 0);
}
/**
* <p>
* Constructs a new MagicNumberFileFilter and associates it with the magic
* number to test for in files. This constructor assumes a starting offset
* of <code>0.
* </p>
*
* Example usage:
* <pre>
* {@code
* MagicNumberFileFilter xmlFileFilter =
* MagicNumberFileFilter("<?xml");
* }
* </pre>
*
* @param magicNumber the magic number to look for in the file.
* The string is converted to bytes using the platform default charset.
*
* @throws IllegalArgumentException if <code>magicNumber is
* <code>null or the empty String.
*/
public MagicNumberFileFilter(String magicNumber) {
this(magicNumber, 0);
}
/**
* <p>
* Constructs a new MagicNumberFileFilter and associates it with the magic
* number to test for in files and the byte offset location in the file to
* to look for that magic number.
* </p>
*
* <code> * MagicNumberFileFilter tarFileFilter =
* MagicNumberFileFilter("ustar", 257);
* </pre>
*
* @param magicNumber the magic number to look for in the file.
* The string is converted to bytes using the platform default charset.
* @param offset the byte offset in the file to start comparing bytes.
*
* @throws IllegalArgumentException if <code>magicNumber is
* <code>null or the empty String, or offset is
* a negative number.
*/
public MagicNumberFileFilter(String magicNumber, long offset) {
if (magicNumber == null) {
throw new IllegalArgumentException("The magic number cannot be null");
}
if (magicNumber.length() == 0) {
throw new IllegalArgumentException("The magic number must contain at least one byte");
}
if (offset < 0) {
throw new IllegalArgumentException("The offset cannot be negative");
}
this.magicNumbers = magicNumber.getBytes(); // uses the platform default charset
this.byteOffset = offset;
}
/**
* <p>
* Constructs a new MagicNumberFileFilter and associates it with the magic
* number to test for in files and the byte offset location in the file to
* to look for that magic number.
* </p>
*
* <p>
* It is important to note that <em>the array is not cloned and that
* any changes to the magic number array after construction will affect the
* behavior of this file filter.
* </p>
*
* <code> * MagicNumberFileFilter tarFileFilter =
* MagicNumberFileFilter(new byte[] {0x75, 0x73, 0x74, 0x61, 0x72}, 257);
* </pre>
*
* <code> * MagicNumberFileFilter javaClassFileFilter =
* MagicNumberFileFilter(new byte[] {0xCA, 0xFE, 0xBA, 0xBE}, 0);
* </pre>
*
* @param magicNumber the magic number to look for in the file.
* @param offset the byte offset in the file to start comparing bytes.
*
* @throws IllegalArgumentException if <code>magicNumber is
* <code>null, or contains no bytes, or offset
* is a negative number.
*/
public MagicNumberFileFilter(byte[] magicNumber, long offset) {
if (magicNumber == null) {
throw new IllegalArgumentException("The magic number cannot be null");
}
if (magicNumber.length == 0) {
throw new IllegalArgumentException("The magic number must contain at least one byte");
}
if (offset < 0) {
throw new IllegalArgumentException("The offset cannot be negative");
}
this.magicNumbers = new byte[magicNumber.length];
System.arraycopy(magicNumber, 0, this.magicNumbers, 0, magicNumber.length);
this.byteOffset = offset;
}
/**
* <p>
* Accepts the provided file if the file contains the file filter's magic
* number at the specified offset.
* </p>
*
* <p>
* If any {@link IOException}s occur while reading the file, the file will
* be rejected.
* </p>
*
* @param file the file to accept or reject.
*
* @return <code>true if the file contains the filter's magic number
* at the specified offset, <code>false otherwise.
*/
@Override
public boolean accept(File file) {
if (file != null && file.isFile() && file.canRead()) {
RandomAccessFile randomAccessFile = null;
try {
byte[] fileBytes = new byte[this.magicNumbers.length];
randomAccessFile = new RandomAccessFile(file, "r");
randomAccessFile.seek(byteOffset);
int read = randomAccessFile.read(fileBytes);
if (read != magicNumbers.length) {
return false;
}
return Arrays.equals(this.magicNumbers, fileBytes);
} catch (IOException ioe) {
// Do nothing, fall through and do not accept file
} finally {
IOUtils.closeQuietly(randomAccessFile);
}
}
return false;
}
/**
* Returns a String representation of the file filter, which includes the
* magic number bytes and byte offset.
*
* @return a String representation of the file filter.
*/
@Override
public String toString() {
StringBuilder builder = new StringBuilder(super.toString());
builder.append("(");
builder.append(new String(magicNumbers));// TODO perhaps use hex if value is not printable
builder.append(",");
builder.append(this.byteOffset);
builder.append(")");
return builder.toString();
}
}
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