可以使用Java的处理Runtime类和ProcessBuilder类来实现安全的打开外部应用程序。

示例代码如下，其中使用ProcessBuilder来创建子进程，并通过指定外部应用程序的绝对路径和参数来运行它。同时，还设置了一个检查外部应用程序运行的超时时间，确保程序不会无限期地挂起。

import java.io.File;
import java.io.IOException;
import java.util.concurrent.TimeUnit;

public class SecureProcessExecution {

    public static void main(String[] args) {
        String externalAppPath = "C:/path/to/external/app.exe";
        String[] externalAppArgs = new String[]{"arg1", "arg2"};

        ProcessBuilder processBuilder = new ProcessBuilder(externalAppPath, externalAppArgs);
        //设置工作目录
        processBuilder.directory(new File("C:/path/to/app/folder"));
        //设置外部应用程序的环境变量
        processBuilder.environment().put("VAR1", "value1");

        try {
            Process process = processBuilder.start();

            //等待子进程运行完成
            boolean finished = process.waitFor(60, TimeUnit.SECONDS);
            if (!finished) {
                //如果运行超时则中止子进程
                process.destroyForcibly();
                throw new RuntimeException("External application timed out");
            }

            //检查子进程的退出代码
            int exitCode = process.exitValue();
            if (exitCode != 0) {
                throw new RuntimeException("External application exited with non-zero code");
            }
        } catch (IOException | InterruptedException e) {
            throw new RuntimeException("Failed to run external application", e);
        }
    }
}