我需要在Java中反复向现有文件追加文本。我怎么做呢?
当前回答
如果你想在特定的行中添加一些文本,你可以先读取整个文件,在你想要的地方追加文本,然后覆盖下面代码中的所有内容:
public static void addDatatoFile(String data1, String data2){
String fullPath = "/home/user/dir/file.csv";
File dir = new File(fullPath);
List<String> l = new LinkedList<String>();
try (BufferedReader br = new BufferedReader(new FileReader(dir))) {
String line;
int count = 0;
while ((line = br.readLine()) != null) {
if(count == 1){
//add data at the end of second line
line += data1;
}else if(count == 2){
//add other data at the end of third line
line += data2;
}
l.add(line);
count++;
}
br.close();
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
createFileFromList(l, dir);
}
public static void createFileFromList(List<String> list, File f){
PrintWriter writer;
try {
writer = new PrintWriter(f, "UTF-8");
for (String d : list) {
writer.println(d.toString());
}
writer.close();
} catch (FileNotFoundException | UnsupportedEncodingException e) {
e.printStackTrace();
}
}
其他回答
稍微扩展一下基普的回答, 下面是一个简单的Java 7+方法来追加一个新行到一个文件,如果它不存在就创建它:
try {
final Path path = Paths.get("path/to/filename.txt");
Files.write(path, Arrays.asList("New line to append"), StandardCharsets.UTF_8,
Files.exists(path) ? StandardOpenOption.APPEND : StandardOpenOption.CREATE);
} catch (final IOException ioe) {
// Add your own exception handling...
}
进一步指出:
The above uses the Files.write overload that writes lines of text to a file (i.e. similar to a println command). To just write text to the end (i.e. similar to a print command), an alternative Files.write overload can be used, passing in a byte array (e.g. "mytext".getBytes(StandardCharsets.UTF_8)). The CREATE option will only work if the specified directory already exists - if it doesn't, a NoSuchFileException is thrown. If required, the following code could be added after setting path to create the directory structure: Path pathParent = path.getParent(); if (!Files.exists(pathParent)) { Files.createDirectories(pathParent); }
这里使用try/catch块的所有答案不都应该在finally块中包含.close()块吗?
有标记的答案示例:
PrintWriter out = null;
try {
out = new PrintWriter(new BufferedWriter(new FileWriter("writePath", true)));
out.println("the text");
} catch (IOException e) {
System.err.println(e);
} finally {
if (out != null) {
out.close();
}
}
此外,从Java 7开始,您可以使用try-with-resources语句。关闭已声明的资源时不需要finally块,因为它是自动处理的,而且不太详细:
try(PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("writePath", true)))) {
out.println("the text");
} catch (IOException e) {
System.err.println(e);
}
您这样做是为了记录日志吗?如果是这样,有几个库。其中最流行的两个是Log4j和Logback。
Java 7 +
对于一次性任务,Files类可以很容易地做到这一点:
try {
Files.write(Paths.get("myfile.txt"), "the text".getBytes(), StandardOpenOption.APPEND);
}catch (IOException e) {
//exception handling left as an exercise for the reader
}
注意:如果文件不存在,上述方法将抛出NoSuchFileException。它也不会自动追加换行符(当追加到文本文件时,您经常希望这样)。另一种方法是同时传递CREATE和APPEND选项,如果文件不存在,则会先创建文件:
private void write(final String s) throws IOException {
Files.writeString(
Path.of(System.getProperty("java.io.tmpdir"), "filename.txt"),
s + System.lineSeparator(),
CREATE, APPEND
);
}
但是,如果要多次写入同一个文件,那么上面的代码段必须多次打开和关闭磁盘上的文件,这是一个缓慢的操作。在这种情况下,BufferedWriter会更快:
try(FileWriter fw = new FileWriter("myfile.txt", true);
BufferedWriter bw = new BufferedWriter(fw);
PrintWriter out = new PrintWriter(bw))
{
out.println("the text");
//more code
out.println("more text");
//more code
} catch (IOException e) {
//exception handling left as an exercise for the reader
}
注:
FileWriter构造函数的第二个参数将告诉它追加到文件中,而不是写入一个新文件。(如果该文件不存在,将创建该文件。) 对于昂贵的写入器(如FileWriter),建议使用BufferedWriter。 使用PrintWriter可以访问您可能在System.out中习惯的println语法。 但是BufferedWriter和PrintWriter包装器并不是严格必要的。
老Java
try {
PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("myfile.txt", true)));
out.println("the text");
out.close();
} catch (IOException e) {
//exception handling left as an exercise for the reader
}
异常处理
如果你需要对旧Java进行健壮的异常处理,它会变得非常冗长:
FileWriter fw = null;
BufferedWriter bw = null;
PrintWriter out = null;
try {
fw = new FileWriter("myfile.txt", true);
bw = new BufferedWriter(fw);
out = new PrintWriter(bw);
out.println("the text");
out.close();
} catch (IOException e) {
//exception handling left as an exercise for the reader
}
finally {
try {
if(out != null)
out.close();
} catch (IOException e) {
//exception handling left as an exercise for the reader
}
try {
if(bw != null)
bw.close();
} catch (IOException e) {
//exception handling left as an exercise for the reader
}
try {
if(fw != null)
fw.close();
} catch (IOException e) {
//exception handling left as an exercise for the reader
}
}
在Java-7中也可以这样做:
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
//---------------------
Path filePath = Paths.get("someFile.txt");
if (!Files.exists(filePath)) {
Files.createFile(filePath);
}
Files.write(filePath, "Text to be added".getBytes(), StandardOpenOption.APPEND);
如果我们使用Java 7及以上版本,并且也知道要添加(追加)到文件的内容,我们可以使用NIO包中的newBufferedWriter方法。
public static void main(String[] args) {
Path FILE_PATH = Paths.get("C:/temp", "temp.txt");
String text = "\n Welcome to Java 8";
//Writing to the file temp.txt
try (BufferedWriter writer = Files.newBufferedWriter(FILE_PATH, StandardCharsets.UTF_8, StandardOpenOption.APPEND)) {
writer.write(text);
} catch (IOException e) {
e.printStackTrace();
}
}
这里有几点需要注意:
指定字符集编码总是一个好习惯,为此我们在StandardCharsets类中有常量。 代码使用try-with-resource语句,其中资源在try之后自动关闭。
虽然OP没有要求,但以防我们想要搜索具有特定关键字的行,例如confidential,我们可以在Java中使用流api:
//Reading from the file the first line which contains word "confidential"
try {
Stream<String> lines = Files.lines(FILE_PATH);
Optional<String> containsJava = lines.filter(l->l.contains("confidential")).findFirst();
if(containsJava.isPresent()){
System.out.println(containsJava.get());
}
} catch (IOException e) {
e.printStackTrace();
}
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