To emulate a **virtual COM port (virtual serial port)** under **Windows**, a software solution needs to interact with the operating system at a low level to create a pair of virtual ports that communicate with each other (like `COM3` ↔ `COM4`). This is typically done using a **kernel-mode driver** or a **user-mode virtual COM port provider**.
### **Approaches to Create a Virtual COM Port in Windows**
1. **Using a Kernel-Mode Driver (Most Reliable)**
- Windows provides the **Serial Port Enumerator (SerCx/Sercx2)** framework for creating virtual COM ports.
- A **Virtual COM Port (VCP) driver** (like `usbser.sys` for USB CDC devices) can be used.
- Third-party tools like **com0com**, **HW VSP3**, or **Eltima Virtual Serial Port Driver** create virtual pairs.
2. **User-Mode Virtual COM Port (via a Provider)**
- Windows allows creating **user-mode COM ports** using the **COM Port Emulator API** (e.g., `CreateFile` with `\\.\` prefix).
- Requires implementing a **COM port provider** that mimics serial port behavior.
---
### **Can Java Create a Virtual COM Port?**
**No, Java cannot directly create a virtual COM port** because:
- Java runs in a **user-mode sandbox** and cannot install kernel drivers.
- Java lacks direct access to Windows' **serial port driver stack**.
However, you can:
1. **Use JNI (Java Native Interface) to call a C/C++ library** that interacts with the Windows COM port API.
2. **Use an existing virtual COM port tool (like com0com) and control it via Java**.
3. **Use a Java library that wraps a virtual COM port solution** (e.g., **RxTx**, **jSerialComm** for existing ports, but not creating new ones).
---
### **Best Workarounds in Java**
1. **Use com0com (Free & Open-Source)**
- Install [com0com](https://sourceforge.net/projects/com0com/) to create virtual COM pairs.
- Control it via Java using `ProcessBuilder`:
```java
ProcessBuilder pb = new ProcessBuilder("command", "to", "configure", "com0com");
Process p = pb.start();
```
- Then use `jSerialComm` to interact with the virtual ports.
2. **Use a Commercial Virtual COM Port Driver (Eltima, HW Group)**
- Some tools provide APIs that Java can call via JNA/JNI.
3. **Simulate a COM Port in User-Mode (Limited Use Case)**
- Create a **TCP-to-Serial bridge** (e.g., using `javax.comm` or `jSerialComm`).
- Example:
```java
// Use jSerialComm to listen on a real/virtual port
SerialPort port = SerialPort.getCommPort("COM3");
port.openPort();
```
---
### **Conclusion**
- **Pure Java cannot create a virtual COM port**, but it can interact with existing ones.
- **Best approach**: Use **com0com** or a **third-party driver** to create virtual ports, then access them via Java (`jSerialComm`).
- For **advanced emulation**, write a **C/C++ driver** and call it via **JNI**.