ProcessBuilder和FileHandler之间的区别

Question

Can any one please explain difference between ProcessBuilder and FileHandler , which one should be used in which situation.

For example if we want to redirect the output of a command to a text file say " logfile.txt ", ProcessBuilder takes few seconds to get the output to the file.

What can this FileHandler do in situations like 100 processes has to send their output to same " logfile.txt "?

Is there a way to get output of all the hundred records to same " logfile.txt " without using process.waitFor() method in ProcessBuilder ? This is my code, but if 100 records are scheduled at same time then p.waitfor() method will not help me because it takes few seconds to execute command and get output to logfile for every process and for 100 processes means it will take more time to execute same commands for all the records in DB. but my application will start for every minute. Thats the problem, process.waitFor() helps me in getting exact output but for more number of records means it takes more time. I want another method where it shouldn't wait but all outputs should append synchronously into file.

while(rs1.next())
    {
        instance_id = rs1.getString(1);
        startdate = rs1.getString(2);
        starttime = rs1.getString(3);
        endtime = rs1.getString(4);
        enddate = rs1.getString(5);
        if(presentdate.equals(startdate) || presentdate.equals(enddate))
        {
            if(presenttime.equals(starttime))
            {
                String[] s1 = new String[]{"cmd", "/c","ec2-start-instances",instance_id,">>","D:\\logfile.log"};
                ProcessBuilder builder1 = new ProcessBuilder(s1);
                Process p1 = builder1.start();
                p1.waitFor();
            }
            else if(presenttime.equals(endtime))
            {
                String[] s1 = new String[]{"cmd", "/c","ec2-stop-instances",instance_id,">>","D:\\logfile.log"};
                ProcessBuilder builder1 = new ProcessBuilder(s1);
                Process p1 = builder1.start();
                p1.waitFor();
            }
        }
    }

Answer 1

For ProcessBuilder according to documentation:

Note that this class is not synchronized. If multiple threads access a ProcessBuilder instance concurrently, and at least one of the threads modifies one of the attributes structurally, it must be synchronized externally.

Example

Process p = new ProcessBuilder("myCommand", "myArg").start();

From my understanding, ProcessBuilder is used to communicate with other software on the OS. (Which could be an outside logging script or some form of logging software)

---

FileHandler is a more of a standard file logging system. However, to avoid the problems from multiple calls, make your logging code static to make sure when it gets called that it finishes it's process. FileHandler flushes by default (according to documentation).

Hope that helps a little bit

Answer 2

They are two mostly unrelated objects and areas.

Yes a Process Builder gives us a standard output and error output stream which can be saved to a file, but it could also be saved to a data base or discarded.

ava.util.logging.FileHandler on the other hand is an appendar part of the logging system. We generally use logging for our own code, though nothing wrong with logging the output of external processes we have called in there too.

But if other classes log while process is logging then will get mixed up logs.

Similarly if your running 50 processes I suggest you dont log to the same file. But log to 50 different files - using 50 threads. Each thread would check the eroror and output streams and save when you have a new line or end of that stream.

When all are done can copy to log file or to a new file as preferred.T

Answer 3

Posting your code actually gives much more idea what is going on. Now I see you are using the windows cmd.exe redirection capabilities instead of reading the outputs from the spawned processes with java and doing something with them. If you like any different behavior, you have to:

1. if possible execute process without cmd.exe (to avoid buffering from it) - new String[]{"ec2-start-instances",instance_id)
2. combine process stdout and stderr (easier to catch all output) processBuilder.redirectErrorStream(true)
3. close process stdin (just a precaution) - process.getOutputStream().close()
4. spawn threads to read all concurrent processes output streams
5. have those threads write what they are reading to a file in a thread safe way

Now #4 and #5 are not trivial but there is plenty of information in the internet how to achieve. In Java 7 you have also the option to redirect to file without any code from your part - processBuilder.redirectOutput(File file) , but I think it is not thread safe so your multiple processes wont play well with each other.

Here are two questions that give some examples how to achieve #4 and #5:

• Write to FileOutputStream from multiple threads in Java
• ProcessBuilder: Forwarding stdout and stderr of started processes without blocking the main thread