问为什么Java命令可以工作，但Runtime.exec不能执行Perforce客户端命令？

EN

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {
    public static void main(String[] args) throws IOException, InterruptedException {
        exec1("p4 label -d mylabel123");
        exec2("p4","label -d mylabel123");
    }
    public static void exec1(String cmd)
            throws java.io.IOException, InterruptedException {
        System.out.println("Executing Runtime.exec()");
        Runtime rt = Runtime.getRuntime();
        Process proc = rt.exec(cmd);

        BufferedReader stdInput = new BufferedReader(new InputStreamReader(
                proc.getInputStream()));
        BufferedReader stdError = new BufferedReader(new InputStreamReader(
                proc.getErrorStream()));

        String s = null;
        while ((s = stdInput.readLine()) != null) {
             System.out.println(s);
        }
        while ((s = stdError.readLine()) != null) {
             System.out.println(s);
        }
        proc.waitFor();
    }
    public static void exec2(String... cmd) throws IOException, InterruptedException{
        System.out.println("\n\nExecuting ProcessBuilder.start()");
        ProcessBuilder pb = new ProcessBuilder();
        pb.inheritIO();
        pb.command(cmd);
        Process process = pb.start();
        process.waitFor();
    }
}