忽略Java4中的变量声明

Question

I have an application which has to be supported in java4 and java5. If it is java 5 execution env i want to add more functionality.

public class Test
{
    private ScheduledExecutorService scheduler;
    private ScheduledFuture<?> scheduledFuture;

    private boolean java4ExecEnv;

    public Test()
    {
        in the manifest.mf file I have something like Bundle-RequiredExecutionEnvironment, using this i am setting java4ExecEnv
        java4ExecEnv = true/false;
    }

    method1()
    {
        if(!java4ExecEnv)
            scheduledFuture = scheduler....
    }

    method2()
    {
        if(!java4ExecEnv)
            scheduledFuture....
    }
}

In the method1 and method2 I can avoid the calls to scheduler and scheduledFuture if it is a java4 version. But is there any way to avoid instance variable declaration(scheduler,scheduledFuture) if the execution environment is java4.

Or should I look for the alternative things which are supported in both java4 and 5.

Answer 1

If it is java 5 execution env i want to add more functionality

Java 5 and Java 1.4 have different APIs. While there are mostly similarities, if you are planning to target a particular version, this has to be done at compile time and not in the code itself. You can use System.getProperty to get the version, but Java is not like C/C++ with conditional #define 's. So if code is compiled on a higher JRE, it's not going to run on a lower JRE.

Answer 2

Here is some demo code that will run on both Java 1.4 and Java 5:

import java.util.concurrent.*;

public class Demo implements Runnable {
  public void run() { System.out.println("Executing"); }

  public static interface Runner { void run(Runnable r); }

  private static class Runner4 implements Runner {
    public void run(Runnable r) {
      System.out.println("Java 1.4");
      new Thread(r).start();
    }
  }

  private static class Runner5 implements Runner {
    ExecutorService executor = Executors.newCachedThreadPool();

    public void run(Runnable r) {
      System.out.println("Java 5");
      executor.submit(r);
    }
  }

  public static Runner loadRunner() {
    try {
      Demo.class.forName("java.util.concurrent.ExecutorService");
      return new Runner5();
    } catch (ClassNotFoundException e) {
      return new Runner4();
    }
  }

  public static void main(String[] args) { loadRunner().run(new Demo()); }
}

This code must be compiled using Java 5 or above with a target version of 1.4. It checks the classpath for the ExecutorService type and falls back to an alternative if it is not present.

If this is a non-trivial project I would make the Java 5 dependent code into some sort of optional plugin and compile the bulk of the code against the Java 1.4 runtime library.