2.7.0版本getContextPath方法：
```
protected String getContextPath(URL url) {
        String contextPath = url.getPath();
        return contextPath.endsWith("/") ? contextPath.substring(0,contextPath.length()-1) : contextPath;
    }
```
无法将方法置入web容器中，退回2.6.5版本的方法：
```
protected String getContextPath(URL url) {
        int pos = url.getPath().lastIndexOf("/");
        return pos > 0 ? url.getPath().substring(0, pos) : "";
    }
```
使用正常，两个方法应该不等价的。
class URL implements Serializable {
    private static final long serialVersionUID = -1985165475234910535L;
    private final String protocol;
    private final String username;
    private final String password;
    
    private final String host;
    
    private final int port;
    private final String path;
    private final Map<String, String> parameters;
    
    private volatile transient Map<String, Number> numbers;
    private volatile transient Map<String, URL> urls;
    private volatile transient String ip;
    private volatile transient String full;
    private volatile transient String identity;
    private volatile transient String parameter;
    private volatile transient String string;
    protected URL() {
        this.protocol = null;
        this.username = null;
        this.password = null;