是否可以在java中动态地"添加"到类路径？

Question

java -classpath ../classes;../jar;. parserTester

如何以编程方式获取上述命令中的功能？喜欢,是否可以运行:

java parserTester

得到相同的结果？我尝试使用URLClassLoader但它修改了类路径并且没有添加它.

感谢名单!

---

感谢Milhous的回应.但这就是我想要做的事情.如何才能首先将jar放入类路径？我也试过使用自定义类加载器:(

这工作..但抱歉,我只需要运行它:java parserTester我想知道这样的事情是否可能???

它需要是如此bcoz我有一个单独的文件夹中的parserTester.java和.class.我需要保留文件结构.parserTester在单独的jar文件夹中使用jar.

Answer 1

您可以使用java.net.URLClassLoader来加载具有您希望的任何程序定义的URL列表的类:

公共类URLClassLoader扩展了SecureClassLoader

此类加载器用于从引用JAR文件和目录的URL的搜索路径加载类和资源.任何以"/"结尾的URL都被假定为引用目录.否则,假定URL引用将根据需要打开的JAR文件.

随后加载类和资源时,将使用创建URLClassLoader实例的线程的AccessControlContext.

默认情况下,加载的类仅被授予访问创建URLClassLoader时指定的URL的权限.

自:1.2

并且一些花哨的步法可以扩展它以支持使用通配路径名来获取JAR的整个目录(这段代码有一些对实用程序方法的引用,但它们的实现在上下文中应该是显而易见的):

/**
 * Add classPath to this loader's classpath.
 * <p>
 * The classpath may contain elements that include a generic file base name.  A generic basename
 * is a filename without the extension that may begin and/or end with an asterisk.  Use of the
 * asterisk denotes a partial match. Any files with an extension of ".jar" whose base name match
 * the specified basename will be added to this class loaders classpath.  The case of the filename is ignored.
 * For example "/somedir/*abc" means all files in somedir that end with "abc.jar", "/somedir/abc*"
 * means all files that start with "abc" and end with ".jar", and "/somedir/*abc*" means all files
 * that contain "abc" and end with ".jar".
 *
 */
public void addClassPath(String cp) {
    String                              seps=File.pathSeparator;                // separators

    if(!File.pathSeparator.equals(";")) { seps+=";"; }                          // want to accept both system separator and ';'
    for(StringTokenizer st=new StringTokenizer(cp,seps,false); st.hasMoreTokens(); ) {
        String pe=st.nextToken();
        File   fe;
        String bn=null;

        if(pe.length()==0) { continue; }

        fe=new File(pe);
        if(fe.getName().indexOf('*')!=-1) {
            bn=fe.getName();
            fe=fe.getParentFile();
            }

        if(!fe.isAbsolute() && pe.charAt(0)!='/' && pe.charAt(0)!='\\') { fe=new File(rootPath,fe.getPath()); }
        try { fe=fe.getCanonicalFile(); }
        catch(IOException thr) {
            log.diagln("Skipping non-existent classpath element '"+fe+"' ("+thr+").");
            continue;
            }
        if(!GenUtil.isBlank(bn)) {
            fe=new File(fe,bn);
            }
        if(classPathElements.contains(fe.getPath())) {
            log.diagln("Skipping duplicate classpath element '"+fe+"'.");
            continue;
            }
        else {
            classPathElements.add(fe.getPath());
            }

        if(!GenUtil.isBlank(bn)) {
            addJars(fe.getParentFile(),bn);
            }
        else if(!fe.exists()) {                                                 // s/never be due getCanonicalFile() above
            log.diagln("Could not find classpath element '"+fe+"'");
            }
        else if(fe.isDirectory()) {
            addURL(createUrl(fe));
            }
        else if(fe.getName().toLowerCase().endsWith(".zip") || fe.getName().toLowerCase().endsWith(".jar")) {
            addURL(createUrl(fe));
            }
        else {
            log.diagln("ClassPath element '"+fe+"' is not an existing directory and is not a file ending with '.zip' or '.jar'");
            }
        }
    log.diagln("Class loader is using classpath: \""+classPath+"\".");
    }

/**
 * Adds a set of JAR files using a generic base name to this loader's classpath.  See @link:addClassPath(String) for
 * details of the generic base name.
 */
public void addJars(File dir, String nam) {
    String[]                            jars;                                   // matching jar files

    if(nam.endsWith(".jar")) { nam=nam.substring(0,(nam.length()-4)); }

    if(!dir.exists()) {
        log.diagln("Could not find directory for Class Path element '"+dir+File.separator+nam+".jar'");
        return;
        }
    if(!dir.canRead()) {
        log.error("Could not read directory for Class Path element '"+dir+File.separator+nam+".jar'");
        return;
        }

    FileSelector fs=new FileSelector(true).add("BaseName","EG",nam,true).add("Name","EW",".jar",true);
    if((jars=dir.list(fs))==null) {
        log.error("Error accessing directory for Class Path element '"+dir+File.separator+nam+".jar'");
        }
    else if(jars.length==0) {
        log.diagln("No JAR files match specification '"+new File(dir,nam)+".jar'");
        }
    else {
        log.diagln("Adding files matching specification '"+dir+File.separator+nam+".jar'");
        Arrays.sort(jars,String.CASE_INSENSITIVE_ORDER);
        for(int xa=0; xa<jars.length; xa++) { addURL(createUrl(new File(dir,jars[xa]))); }
        }
    }

private URL createUrl(File fe) {
    try {
        URL url=fe.toURI().toURL();
        log.diagln("Added URL: '"+url.toString()+"'");
        if(classPath.length()>0) { classPath+=File.pathSeparator; }
        this.classPath+=fe.getPath();
        return url;
        }
    catch(MalformedURLException thr) {
        log.diagln("Classpath element '"+fe+"' could not be used to create a valid file system URL");
        return null;
        }
    }

Answer 2

您可以实现自己的类加载器，但该类/jar 必须位于类路径中才能执行。

尝试

java -cp *.jar:. myClass

或者

export CLASSPATH=./lib/tool.jar:.
java myClass

或者

java -jar file.jar