以静态和非静态方法返回相同的生成的UUID

Question

I have an object which is defined like so:

TestDriver.java

public void TestDriver extends FirefoxDriver implements SupportDownloads{

    public TestDriver(){
        super(TestDriver.service(), TestDriver.options());
    }

    private static service(){
        // implementation
    }

    private static options(){
        ..
        // attempting to generate a UUID which will be eventually be set as the folder where all downloads will go to. 
        // unique paths are needed as we will be using this class for multi-threaded runs and would like each browser to have its own unique download folder
        byte[] value = String.valueOf(Thread.currentThread().getId()).getBytes());
        String uniqueID = UUID.nameUUIDFromBytes(value).toString();
        ..
    }

    // purpose of this method is to return the path download folder 
    // note the repeated generation of the UUID because the above methods are static
    @Override
    public File getDownloadPath(){
        String value = String.valueOf(Thread.currentThread().getId());
        return new File(prop.getProperty(fileDirectory()) + File.separator + UUID.nameUUIDFromBytes(value.getBytes()));    
    }
}

SupportsDownload.java

public interface SupportsDownload {
    public File getDownloadPath();
}

What I am trying to do is:

1. To generate a UUID so that each thread created have unique download paths
2. To be able to also return this UUID in a public method which is accessible outside of this class (for unit-testing purposes and usage by the users)

Even though the above code is already working, I will run into an issue which official Java docs mention:

Returns the identifier of this Thread. The thread ID is a positive long number generated when this thread was created. The thread ID is unique and remains unchanged during its lifetime. When a thread is terminated, this thread ID may be reused.

As such, the above method is not foolproof because basing the UUID seed only on Thread.currentThread().getId() may yield the same ID if it is reused, causing the UUID generated to be the same.

I then tried to modify it to include time in millis, ie:

        byte[] value = String.valueOf(Thread.currentThread().getId() + Calendar.getInstance().getTimeInMillis()).getBytes());

However, as time value always changes, I wouldn't be able to grab the exact same value in the return method getDownloadPath() .

If I were to generate the time value once, and store it in a variable, ie:

private final long timeInMillis = Calendar.getTime().getTimeInMillis();

I will not be able to utilize this variable for both the static and non-static methods.

I'm at a loss, any suggestions would be great :-)

Answer 1

I believe you should use ThreadLocal to do your task. Below I have added a sample code.

public class Main {

    public static void main(String[] args) {
        Thread t1 = new Thread(() -> {
            MyUUIDFactory.getUUIDPerThread();
            MyUUIDFactory.getUUIDPerThread();
        }, "My Thread-1");

        Thread t2 = new Thread(() -> {
            MyUUIDFactory.getUUIDPerThread();
            MyUUIDFactory.getUUIDPerThread();
        }, "My Thread-2");

        t1.start();
        t2.start();
    }

}

class MyUUIDFactory {
    private static final ThreadLocal<String> localWebDriver = ThreadLocal.withInitial(
            () -> UUID.randomUUID().toString());

    public static String getUUIDPerThread() {
        return localWebDriver.get();
    }
}