com.sun.*, sun.* 开头的包都是 JRE 的底层类库，未被归档化，这是由于这些类都是依赖于平台的，在 Windows 平台上有这个类，在 Linux 上可能就没有这个类。
我们应该尽量少用 JRE 的底层类库：http://www.oracle.com/technetwork/java/faq-sun-packages-142232.html
The java.*, javax.* and org.* packages documented in the Java Platform Standard Edition API Specification make up the official, supported, public interface.
The sun.* packages are not part of the supported, public interface.
In general, writing java programs that rely on sun.* is risky: those classes are not portable, and are not supported.
原文：http://my.oschina.net/JustLoveIT/blog/471928