我对Class的标识感到怀疑,Class的标识应该是The same classloader instance + The same class full path。但是我做了一些测试用例,这是行不通的。

我有一个自定义的ClassLoader:

import java.io.*;

public class FileSystemClassLoader extends ClassLoader {

    private String rootDir;

    public FileSystemClassLoader(String rootDir) {
        this.rootDir = rootDir;
    }

    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] classData = getClassData(name);
        if (classData == null) {
            throw new ClassNotFoundException();
        } else {
            return defineClass(name, classData, 0, classData.length);
        }
    }

    private byte[] getClassData(String className) {
        String path = classNameToPath(className);
        try {
            InputStream ins = new FileInputStream(path);
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            int bufferSize = 4096;
            byte[] buffer = new byte[bufferSize];
            int bytesNumRead = 0;
            while ((bytesNumRead = ins.read(buffer)) != -1) {
                baos.write(buffer, 0, bytesNumRead);
            }
            return baos.toByteArray();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

    private String classNameToPath(String className) {
        return rootDir + File.separatorChar
                + className.replace('.', File.separatorChar) + ".class";
    }
}


Sample类:

public class Sample {

    private Sample instance;

    public void setSample(Object instance) {
        this.instance = (Sample) instance;
    }
}


还有一个测试用例:

String classDataRootPath = "/Users/haolin/Github/jvm/target/classes";
FileSystemClassLoader fscl1 = new    FileSystemClassLoader(classDataRootPath);
FileSystemClassLoader fscl2 = new FileSystemClassLoader(classDataRootPath);
String className = "me.hao0.jvm.classloader.Sample";
try {
    Class<?> class1 = fscl1.loadClass(className);
    Object obj1 = class1.newInstance();
    Class<?> class2 = fscl2.loadClass(className);
    Object obj2 = class2.newInstance();
    Method setSampleMethod = class1.getMethod("setSample", Object.class);
    setSampleMethod.invoke(obj1, obj2);
} catch (Exception e) {
    e.printStackTrace();
}


ClassCastException时应在setSampleMethod.invoke(obj1, obj2)发生,因为obj1和obj2是不同的Class(它们的ClassLoader是不同的),但是代码运行良好,不会抛出ClassCastException

有人可以建议吗?

最佳答案

除了一个重要的细节,您将是正确的。类加载器存在于层次结构中,每个ClassLoader在尝试加载类本身之前都会委派给其父级。因此,如果您的类加载器的共同祖先找到了所请求的类,那么您的类加载器将都返回该类的相同实例。考虑以下示例:

public class Foo {
    public void isFoo(Object obj) {
        System.out.println("object is a Foo: " + (obj instanceof Foo));
        Foo foo = (Foo) obj;
    }
}


和测试:

import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;

public class Test {
    public static void main(String[] args) throws Exception {
        File path = new File(".");
        URL[] urls = new URL[] { path.toURI().toURL() };
        URLClassLoader cl1 = new URLClassLoader(urls);
        URLClassLoader cl2 = new URLClassLoader(urls);

        Class c1 = cl1.loadClass("Foo");
        Class c2 = cl2.loadClass("Foo");
        System.out.println("same class instance: " + (c1 == c2));

        Object o1 = c1.newInstance();
        Object o2 = c2.newInstance();

        Method m = c1.getDeclaredMethod("isFoo", Object.class);
        m.invoke(o1, o2);
    }
}


输出为:

same class instance: true
object is a Foo: true


发生这种情况是因为当前目录是默认类路径的一部分,所以父类加载器查找并加载Foo,并且两个自定义类加载器均从其父级返回实例。

现在,对测试类进行此更改并重新编译:

File path = new File("foo");


创建foo/目录,然后将Foo.class移至该目录。现在的输出是:

same class instance: false
object is a Foo: false
Exception in thread "main" java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at Test.main(Test.java:21)
Caused by: java.lang.ClassCastException: Foo cannot be cast to Foo
    at Foo.isFoo(Foo.java:4)
    ... 5 more


这就是您所期望的。系统类加载器无法再找到Foo,因此自定义类加载器将加载它的单独实例。 JVM将两个Class实例视为不同的类,即使它们是相同的,并且强制转换失败。

关于java - java类标识有关classloader,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/36440731/

10-11 20:53