由 ClassLoader 加载的沙盒类

ClassLoader 需要提供识别代码源的 ProtectionDomain：

public class PluginClassLoader extends ClassLoader {
    private final ClassProvider provider;

    private final ProtectionDomain pd;

    public PluginClassLoader(ClassProvider provider) {
        this.provider = provider;
        Permissions permissions = new Permissions();
        
        this.pd = new ProtectionDomain(provider.getCodeSource(), permissions, this, null);
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] classDef = provider.getClass(name);
        Class<?> clazz = defineClass(name, classDef, 0, classDef.length, pd);
        return clazz;
    }
}

通过重写 findClass 而不是 loadClass，保留委托模型，PluginClassLoader 将首先查询系统和父类加载器以获取类定义。

制定政策：

public class PluginSecurityPolicy extends Policy {
    private final Permissions appPermissions = new Permissions();
    private final Permissions pluginPermissions = new Permissions();

    public PluginSecurityPolicy() {
        // amend this as appropriate
        appPermissions.add(new AllPermission());
        // add any permissions plugins should have to pluginPermissions
    }

    @Override
    public Provider getProvider() {
        return super.getProvider();
    }

    @Override
    public String getType() {
        return super.getType();
    }

    @Override
    public Parameters getParameters() {
        return super.getParameters();
    }

    @Override
    public PermissionCollection getPermissions(CodeSource codesource) {
        return new Permissions();
    }

    @Override
    public PermissionCollection getPermissions(ProtectionDomain domain) {
        return isPlugin(domain)?pluginPermissions:appPermissions;
    }

    private boolean isPlugin(ProtectionDomain pd){
        return pd.getClassLoader() instanceof PluginClassLoader;
    }

}

最后，设置策略和 SecurityManager（默认实现很好）：

    Policy.setPolicy(new PluginSecurityPolicy());
    System.setSecurityManager(new SecurityManager());