我之前已经问过这个问题:
How to use URLClassLoader to load a *.class file?
但由于缺乏榜样,我不太懂。我目前正在处理一个项目并试图加载用户给定的.class对象,这些对象可以位于机器上的任何目录中。
//Create URL to hash function class file
URL url_hashFunctionPath = new URL("file:///" + _sHashFunctionFilePath);
//Packet URL to a URL array to be used by URLClassLoader
URL[] urlA_hashFunctionPath = {url_hashFunctionPath};
//Load URL for hash function via a URL class loader
URLClassLoader urlCl_hashFunctionClassLoader = new URLClassLoader(urlA_hashFunctionPath);
//Load user hash function into class to initialize later (TEST: HARD CODE FOR NOW)
m_classUserHashClass = urlCl_hashFunctionClassLoader.loadClass(_sUserHashClassName);
最后一行给了我一个ClassNotFoundException,来自我的实验&理解用户给定的类函数必须在类路径中?
PS:第一次发帖问题随时可以纠正我没有遵循适当方式的地方。
//解
我在[WillShackleford] [1]的慷慨帮助下得到的解决方案,此解决方案可以在给定的文件路径中加载.class文件。有关更多信息,请参阅代码及其给出的注释。
//The absolute file path to the class that is to be loaded (_sHashFunctionFilePath = absolute file path)
String pathToClassFile = _sHashFunctionFilePath;
System.out.println("File to class: " + _sHashFunctionFilePath);
//Declare the process builder to execute class file at run time (Provided filepath to class)
ProcessBuilder pb = new ProcessBuilder("javap", pathToClassFile);
try
{
//Start the process builder
Process p = pb.start();
//Declare string to hold class name
String classname = null;
//Declare buffer reader to read the class file & get class name
try(BufferedReader br = new BufferedReader(new InputStreamReader(p.getInputStream())))
{
String line;
while(null != (line = br.readLine()))
{
if(line.startsWith("public class"))
{
classname = line.split(" ")[2];
break;
}
}
System.out.println("classname = " + classname);
}
catch(IOException _error)
{
}
//Declare file path to directory containing class to be loaded
String pathToPackageBase = pathToClassFile.substring(0, pathToClassFile.length() - (classname + ".class").length());
System.out.println("pathToPackageBase = " + pathToPackageBase);
try
{
//Create class to hold the class to be loaded via a URL class loader
Class clss = new URLClassLoader(
new URL[]{new File(pathToPackageBase).toURI().toURL()}
).loadClass(classname);
//Create ab object/instance of said class
Object test = clss.newInstance();
//Declare & create requested method from user hash function class (Method is work & takes no arguments)
Method method = clss.getMethod("work", null);
method.invoke(test, null);
}
答案 0 :(得分:3)
在目录/home/shackle/somedir/classes/pkg
中,我有一个文件Test.class,它是从带有package pkg;
的java文件创建的,例如:
package pkg;
public class Test {
public String toString() {
return "secret_string";
}
}
然后我加载它:
System.out.println(new URLClassLoader(
new URL[]{new File("/home/shackle/somedir/classes").toURI().toURL()}
).loadClass("pkg.Test").newInstance().toString());
请注意,我没有将pkg / Test放在URL字符串中,但是load class参数具有pkg。前缀。
您可以直接从文件中获取类名:
Class clsReaderClss = ClassLoader.getSystemClassLoader().loadClass("jdk.internal.org.objectweb.asm.ClassReader");
System.out.println("clsReaderClss = " + clsReaderClss);
Constructor con = clsReaderClss.getConstructor(InputStream.class);
Object reader = con.newInstance(new FileInputStream(directFile));
Method m = clsReaderClss.getMethod("getClassName");
String name = m.invoke(reader).toString().replace('/', '.');
System.out.println("name = " + name);
不需要访问内部类的替代方案。
String pathToClassFile = "/home/shackle/somedir/classes/pkg/Test.class";
ProcessBuilder pb = new ProcessBuilder("javap",pathToClassFile);
Process p = pb.start();
String classname = null;
try(BufferedReader br = new BufferedReader(new InputStreamReader(p.getInputStream()))) {
String line;
while(null != (line = br.readLine())) {
if(line.startsWith("public class")) {
classname = line.split(" ")[2];
break;
}
}
}
System.out.println("classname = " + classname);
然后可以加载类:
String pathToPackageBase = pathToClassFile.substring(0, pathToClassFile.length() - (classname + ".class").length());
System.out.println("pathToPackagBase = " + pathToPackageBase);
Class clss = new URLClassLoader(
new URL[]{new File(pathToPackageBase).toURI().toURL()}
).loadClass(classname);
System.out.println(clss.newInstance().toString());
答案 1 :(得分:0)
您的_sHashFunctionFilePath
需要从中删除目标类的包名称,因此ClassLoader会在_sHashFunctionFilePath
+ package.name
+ HashFunction.class
中查找文件。如果不这样做,ClassLoader将无法找到该文件。
因此,如果目标类在my.great.HashFunction
中为HashFunction.class
,则如果要使用URLClassLoader,则需要位于名为my/great/
的目录中。然后,如果在/path/to
中实际找到.class文件,则使用file:///
作为URLClassLoader的/path/to/my/great/HashFunction.class
网址。