我想知道使用什么技术和/或库来实现linux命令“ tail -f”的功能。我本质上是在寻找附加组件/替换的下降java.io.FileReader。客户端代码可能如下所示：
TailFileReader lft = new TailFileReader("application.log");
BufferedReader br = new BufferedReader(lft);
String line;
try {
while (true) {
line= br.readLine();
// do something interesting with line
}
} catch (IOException e) {
// barf
}
缺少的部分是的合理实现TailFileReader。它应该能够读取文件打开之前存在的部分以及添加的行。
Answers:
能够继续读取文件并等待文件有更多更新的能力，自己编写代码并不难。这是一些伪代码：
BufferedReader br = new BufferedReader(...);
String line;
while (keepReading) {
line = reader.readLine();
if (line == null) {
//wait until there is more of the file for us to read
Thread.sleep(1000);
}
else {
//do something interesting with the line
}
}
我假设您想将这种功能放在其自己的线程中，以便可以使其休眠而不影响应用程序的任何其他区域。您可能希望公开keepReading一个setter，以便您的主类/应用程序的其他部分可以安全地关闭线程而不会造成其他麻烦，只需调用stopReading()或类似方法即可。
检查一下 JLogTailer，它执行此逻辑。
该代码的要点是：
public void run() {
try {
while (_running) {
Thread.sleep(_updateInterval);
long len = _file.length();
if (len < _filePointer) {
// Log must have been jibbled or deleted.
this.appendMessage("Log file was reset. Restarting logging from start of file.");
_filePointer = len;
}
else if (len > _filePointer) {
// File must have had something added to it!
RandomAccessFile raf = new RandomAccessFile(_file, "r");
raf.seek(_filePointer);
String line = null;
while ((line = raf.readLine()) != null) {
this.appendLine(line);
}
_filePointer = raf.getFilePointer();
raf.close();
}
}
}
catch (Exception e) {
this.appendMessage("Fatal error reading log file, log tailing has stopped.");
}
// dispose();
}
前一段时间，我在Scala中构建了一个简短的“ tail -f”实现：tailf。它也负责文件的轮换，您可以定义自己的逻辑，当文件到达EOF或发现文件已重命名时该怎么做。
您可以看一下并将其移植到Java，因为实际上那里没有什么复杂的东西。一些注意事项：主文件是Tail.scala，基本上它定义FollowingInputStream了EOF /重命名和follow方法，将其包装FollowingInputStream到中的无限制枚举中SequenceInputStream。因此，一旦FollowingInputStream结束，就会SequenceInputStream从Enumeration和另一个请求FollowingInputStream创建下一个元素。
我最近偶然发现了rxjava-file，它是RxJava的扩展。与其他解决方案相比，它利用了Java的NIO。
import rx.Observable;
import rx.functions.Action1;
import com.github.davidmoten.rx.FileObservable;
// ... class definition omitted
public void tailLogFile() throws InterruptedException {
Observable<String> tailer = FileObservable.tailer()
.file("application.log") // absolute path
.tailText();
tailer.subscribe(
new Action1<String>() {
@Override
public void call(String line) {
System.out.println("you got line: " + line);
}
},
new Action1<Throwable>() {
@Override
public void call(Throwable e) {
System.out.println("you got error: " + e);
e.printStackTrace();
}
}
);
// this solution operates threaded, so something
// is required that prevents premature termination
Thread.sleep(120000);
}
如果您的代码只必须在Unix系统上运行，那么您只需脱壳并tail -f直接调用就可以摆脱困境。
作为更复杂的替代方案，您可以看看GNU尾部的实现并将其移植到Java。（不过，我不确定这是否还不能使您的代码成为派生作品。）
刚遇到同样的问题-在这里找到“最简单”的实现：Java尾巴。
*好东西* -准备生产;）
我希望引用代码不会丢掉一些许可证。
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
/**
* Java implementation of the Unix tail command
*
* @param args[0] File name
* @param args[1] Update time (seconds). Optional. Default value is 1 second
*
* @author Luigi Viggiano (original author) http://it.newinstance.it/2005/11/19/listening-changes-on-a-text-file-unix-tail-implementation-with-java/
* @author Alessandro Melandri (modified by)
* */
public class Tail {
static long sleepTime = 1000;
public static void main(String[] args) throws IOException {
if (args.length > 0){
if (args.length > 1)
sleepTime = Long.parseLong(args[1]) * 1000;
BufferedReader input = new BufferedReader(new FileReader(args[0]));
String currentLine = null;
while (true) {
if ((currentLine = input.readLine()) != null) {
System.out.println(currentLine);
continue;
}
try {
Thread.sleep(sleepTime);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
break;
}
}
input.close();
} else {
System.out.println("Missing parameter!\nUsage: java JavaTail fileName [updateTime (Seconds. default to 1 second)]");
}
}
}
我发现这个不错的尾巴实现。
作者：amelandri
源于：https ://gist.github.com/amelandri/1376896
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
/**
* Java implementation of the Unix tail command
*
* @param args[0] File name
* @param args[1] Update time (seconds). Optional. Default value is 1 second
*
* @author Luigi Viggiano (original author) http://it.newinstance.it/2005/11/19/listening-changes-on-a-text-file-unix-tail-implementation-with-java/
* @author Alessandro Melandri (modified by)
* */
public class Tail {
static long sleepTime = 1000;
public static void main(String[] args) throws IOException {
if (args.length > 0){
if (args.length > 1)
sleepTime = Long.parseLong(args[1]) * 1000;
BufferedReader input = new BufferedReader(new FileReader(args[0]));
String currentLine = null;
while (true) {
if ((currentLine = input.readLine()) != null) {
System.out.println(currentLine);
continue;
}
try {
Thread.sleep(sleepTime);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
break;
}
}
input.close();
} else {
System.out.println("Missing parameter!\nUsage: java JavaTail fileName [updateTime (Seconds. default to 1 second)]");
}
}
}