BOM——Byte Order Mark，就是字节序标记。
基本概念
- 在
UCS 编码中有一个叫做”ZERO WIDTH NO-BREAK SPACE“的字符，它的编码是FEFF。而FFFE在UCS中是不存在的字符，所以不应该出现在实际传输中。 - UCS规范建议我们在传输字节流前，先传输 字符”
ZERO WIDTH NO-BREAK SPACE“。 - 如果接收者收到
FEFF，就表明这个字节流是大字节序的；如果收到FFFE，就表明这个字节流是小字节序的。因此字符”ZERO WIDTH NO-BREAK SPACE“又被称作BOM。
UTF-8不需要BOM来表明字节顺序，但可以用BOM来表明编码方式。字符”ZERO WIDTH NO-BREAK SPACE“的UTF-8编码是EF BB BF。所以如果接收者收到以EF BB BF开头的字节流，就知道这是UTF-8编码了。
这个BOM头部对于UTF-8来说不是必须的，并且我建议最好不用有这个头部，以避免可能的兼容性问题。
下面就来看看怎么用java来处理UTF-8的BOM头部
增加BOM到UTF-8文件
import java.io.BufferedWriter;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
public class AddBomToUtf8File {
public static void main(String[] args) throws IOException {
Path path = Paths.get("/home/file.txt");
writeBomFile(path, "billy");
}
private static void writeBomFile(Path path, String content) {
// Java 8 default UTF-8
try (BufferedWriter bw = Files.newBufferedWriter(path)) {
bw.write("\ufeff");
bw.write(content);
bw.newLine();
bw.write(content);
} catch (IOException e) {
e.printStackTrace();
}
}
}
在Java 8 之前可以用下面的方法：
new OutputStreamWriter(
new FileOutputStream(path.toFile())
, StandardCharsets.UTF_8))) {
bw.write("\ufeff");
bw.write(content);
bw.newLine();
bw.write(content);
} catch (IOException e) {
e.printStackTrace();
}
}
或者可以用 PrintWriter 和OutputStreamWriter
try (PrintWriter pw = new PrintWriter(
new OutputStreamWriter(
new FileOutputStream(path.toFile()), StandardCharsets.UTF_8))) {
//pw.write("\ufeff");
pw.write(0xfeff); // alternative, codepoint
pw.write(content);
pw.write(System.lineSeparator());
pw.write(content);
} catch (IOException e) {
e.printStackTrace();
}
}
又或者，这样:
private static void writeBomFile4(Path path, String content) {
try (FileOutputStream fos = new FileOutputStream(path.toFile())) {
byte[] BOM = {(byte) 0xEF, (byte) 0xBB, (byte) 0xBF};
fos.write(BOM);
fos.write(content.getBytes(StandardCharsets.UTF_8));
fos.write(System.lineSeparator().getBytes(StandardCharsets.UTF_8));
fos.write(content.getBytes(StandardCharsets.UTF_8));
} catch (IOException e) {
e.printStackTrace();
}
}
检查文件是否包含BOM头部
import java.io.IOException;
import java.io.InputStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
public class CheckBom {
public static void main(String[] args) throws IOException {
Path path = Paths.get("/home/file.txt");
if(isContainBOM(path)){
System.out.println("Found BOM!");
}else{
System.out.println("No BOM.");
}
}
private static boolean isContainBOM(Path path) throws IOException {
if(Files.notExists(path)){
throw new IllegalArgumentException("Path: " + path + " does not exists!");
}
boolean result = false;
byte[] bom = new byte[3];
try(InputStream is = new FileInputStream(path.toFile())){
// read first 3 bytes of a file.
is.read(bom);
// BOM encoded as ef bb bf
String content = new String(Hex.encodeHex(bom));
if ("efbbbf".equalsIgnoreCase(content)) {
result = true;
}
}
return result;
}
}
上面的代码需要一个依赖：
<dependency>
<groupId>commons-codec</groupId>
<artifactId>commons-codec</artifactId>
<version>1.14</version>
</dependency>
移除UTF-8文件的BOM头部
通常，我建议不要用这个BOM，不然处理不好产生什么乱码就麻烦了。
import org.apache.commons.codec.binary.Hex;
import java.io.BufferedWriter;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.nio.ByteBuffer;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
public class RemoveBomFromUtf8File {
public static void main(String[] args) throws IOException {
Path path = Paths.get("/home/file.txt");
writeBomFile(path, "billy");
removeBom(path);
}
private static void writeBomFile(Path path, String content) {
// Java 8 default UTF-8
try (BufferedWriter bw = Files.newBufferedWriter(path)) {
bw.write("\ufeff");
bw.write(content);
bw.newLine();
bw.write(content);
} catch (IOException e) {
e.printStackTrace();
}
}
private static boolean isContainBOM(Path path) throws IOException {
if (Files.notExists(path)) {
throw new IllegalArgumentException("Path: " + path + " does not exists!");
}
boolean result = false;
byte[] bom = new byte[3];
try (InputStream is = new FileInputStream(path.toFile())) {
// read 3 bytes of a file.
is.read(bom);
// BOM encoded as ef bb bf
String content = new String(Hex.encodeHex(bom));
if ("efbbbf".equalsIgnoreCase(content)) {
result = true;
}
}
return result;
}
private static void removeBom(Path path) throws IOException {
if (isContainBOM(path)) {
byte[] bytes = Files.readAllBytes(path);
ByteBuffer bb = ByteBuffer.wrap(bytes);
System.out.println("Found BOM!");
byte[] bom = new byte[3];
// get the first 3 bytes
bb.get(bom, 0, bom.length);
// remaining
byte[] contentAfterFirst3Bytes = new byte[bytes.length - 3];
bb.get(contentAfterFirst3Bytes, 0, contentAfterFirst3Bytes.length);
System.out.println("Remove the first 3 bytes, and overwrite the file!");
// override the same path
Files.write(path, contentAfterFirst3Bytes);
} else {
System.out.println("This file doesn't contains UTF-8 BOM!");
}
}
}
复制UTF-8文件并追加BOM
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
public class CopyAndAddBomToXmlFile {
public static void main(String[] args) {
Path src = Paths.get("src/main/resources/staff.xml");
Path dest = Paths.get("src/main/resources/staff-bom.xml");
writeBomFile(src, dest);
}
private static void writeBomFile(Path src, Path dest) {
try (FileOutputStream fos = new FileOutputStream(dest.toFile())) {
byte[] BOM = {(byte) 0xEF, (byte) 0xBB, (byte) 0xBF};
// add BOM
fos.write(BOM);
// BOM + src to fos
Files.copy(src, fos);
} catch (IOException e) {
e.printStackTrace();
}
}
}
