Java包含多个库，在运行时决定使用哪一个？

Question

I want to change the serial library of my application from purejavacomm to jRxTx . This should be relatively easy since they have very similar functionality.

But since my application with purejavacomm is well tested and trusted by my customers and with jRxTx it is not I want to include both libraries in my code.

So when I see a problem with the jRxTx implementation in a customer system I can just delete the jRxTx.jar from the customer PC and it automatically uses purejavacomm again.

I wanted to write a small test programm for this to see if I could just catch the NoClassDefFoundError when the jRxTx.jar is not there and use the purejavacomm instead but that does not seem to work.

     try {
            try {
                gnu.io.SerialPort jrxtxSerialPort;

                gnu.io.CommPortIdentifier portIdentifier = gnu.io.CommPortIdentifier.getPortIdentifier("COM12");

                jrxtxSerialPort = (gnu.io.SerialPort) portIdentifier.open("Test", 0);
                jrxtxSerialPort.setFlowControlMode(SerialPort.FLOWCONTROL_NONE);
                jrxtxSerialPort.setSerialPortParams(19200, 8, 1, SerialPort.PARITY_EVEN);
                jrxtxSerialPort.getOutputStream().write("TEST\n".getBytes());
                System.out.println("\n" + readFromInputStream(jrxtxSerialPort.getInputStream()) + "\n\n");
                jrxtxSerialPort.close();
                jRxTxLibFound = true;
            } catch (java.io.IOException e) {
                e.printStackTrace();
            } catch (gnu.io.NoSuchPortException e) {
                e.printStackTrace();
            } catch (gnu.io.PortInUseException e) {
                e.printStackTrace();
            } catch (gnu.io.UnsupportedCommOperationException e) {
                e.printStackTrace();
            }
        } catch (java.lang.NoClassDefFoundError e) {
            System.out.println("jRxTx not found. Try with purejavacomm");
        }

I expected that this code would catch the NoClassDefFoundError but I still get the NoClassDefFoundError:

java.lang.NoClassDefFoundError: gnu/io/NoSuchPortException
    at java.lang.Class.getDeclaredMethods0(Native Method)
    at java.lang.Class.privateGetDeclaredMethods(Class.java:2701)
    at java.lang.Class.privateGetMethodRecursive(Class.java:3048)
    at java.lang.Class.getMethod0(Class.java:3018)
    at java.lang.Class.getMethod(Class.java:1784)
    at sun.launcher.LauncherHelper.validateMainClass(LauncherHelper.java:544)
    at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:526)
Caused by: java.lang.ClassNotFoundException: gnu.io.NoSuchPortException
    at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    ... 7 more
Error: A JNI error has occurred, please check your installation and try again
Exception in thread "main"

Answer 1

I solved this problem by writing wrapper classes for each library, which implement a common interface. This Interface has a static getInstance() method, which looks in the classpath for the jRxTx.jar file and if it doesn't find it the interface returns the purejavacomm class.

String classpathStr = System.getProperty("java.class.path");
if (classpathStr.contains("jrxtx.1.0.1.jar")) {
    return new JRxTxCommPortIdentifier();
} else {
    return new PureJavaCommCommPortIdentifier();
}

JRxTxCommPortIdentifier and PureJavaCommCommPortIdentifier are two of the wrapper classes.

If you know of any more elegant ways to solve this please let me know.