在运行时加载 jars [英] Loading jars at runtime

问题描述

我正在尝试在运行时将 jar 文件添加到类路径.我用这个代码

I am trying to add jar file to classpath at runtime. I use this code

 public static void addURL(URL u) throws IOException {

            URLClassLoader sysloader = (URLClassLoader) ClassLoader
                    .getSystemClassLoader();
            Class<URLClassLoader> sysclass = URLClassLoader.class;

            try {
                Method method = sysclass.getDeclaredMethod("addURL", parameters);
                method.setAccessible(true);
                method.invoke(sysloader, new Object[] { u });
                System.out.println(u);
            } catch (Throwable t) {
                t.printStackTrace();
                throw new IOException("Error");
            }

        }

系统输出打印这个网址:

System out prints this url:

file:/B:/Java/Tools/mysql-connector-java-5.1.18/mysql-connector-java-5.1.18/mysql-connector-java-5.1.18-bin.jar

我仔细检查了这条路径，这个 jar 存在.连这个测试都显示com.mysql.jdbc.驱动程序类存在.

I was check this path carefully, this jar exist. Even this test show that com.mysql.jdbc. Driver class exists.

javap -classpath "B:JavaToolsmysql-connector-java-5.1.18
mysql-connector-java-5.1.18mysql-connector-java-5.1.18-bin.jar" com.mysql.jdbc.
Driver
Compiled from "Driver.java"
public class com.mysql.jdbc.Driver extends com.mysql.jdbc.NonRegisteringDriver i
mplements java.sql.Driver{
    public com.mysql.jdbc.Driver()       throws java.sql.SQLException;
    static {};
}

但是当我使用这个 Class.forName(driver) 时，我仍然得到 java.lang.ClassNotFoundException.这段代码有什么问题?

But I still get java.lang.ClassNotFoundException when I use this Class.forName(driver). What is wrong with this code?

推荐答案

URL 没问题，但是您尝试从 classpath 加载 jar，因此这意味着您需要先将文件放在 cp 中.在您的情况下，您想加载一个不在类路径中的 jar，因此您必须使用URLClassLoader 和 JAR 你也可以使用 JARClassLoader如果您想要一些示例课程:http://docs.oracle.com/javase/tutorial/deployment/jar/jarclassloader.html

The URL is ok, nevertheless you try to load a jar from classpath, so it means that yo need to have the file in cp first. In your case you want to load a jar that is not in classpath so you have to use URLClassLoader and for JAR you can use also the JARClassLoader If you want some sample lesson on it: http://docs.oracle.com/javase/tutorial/deployment/jar/jarclassloader.html

这是我自己运行的示例，看看是否对您有帮助.它搜索不在我的类路径中的 Log4j 的 Logger 类，当然我在调用构造函数时遇到了异常，因为我没有将正确的参数传递给构造函数

Here a sample I ran by myself see if helps you. It search the Logger class of Log4j that is not in my classpath, of course i got exception on invocation of the constructor since i did not pass the right params to the constructor

package org.stackoverflow;
import java.io.File;
import java.net.URL;
import java.net.URLClassLoader;

public class URLClassLoaderSample
{
  public static void main(String[] args) throws Exception
  {
    File f = new File("C:\_programs\apache\log4j\v1.1.16\log4j-1.2.16.jar");
    URLClassLoader urlCl = new URLClassLoader(new URL[] { f.toURL()},System.class.getClassLoader());
    Class log4jClass = urlCl.loadClass("org.apache.log4j.Logger");
    log4jClass.newInstance();
  }
}



Exception in thread "main" java.lang.InstantiationException: org.apache.log4j.Logger
    at java.lang.Class.newInstance0(Class.java:357)
    at java.lang.Class.newInstance(Class.java:325)
    at org.stackoverflow.URLClassLoaderSample.main(URLClassLoaderSample.java:19)

由于错误调用导致的异常，不过在这个阶段我们已经找到了类

Exception due to the wrong invocation, nevertheless at this stage we already found the class

这篇关于在运行时加载 jars的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！