Subject: TraceContext class is not found

Hi, Sheng,

I run a app with agent on my pc and got a ClassNotFoundException while using the 'apm-toolkit-trace(7.0.0)' library. This is a spriing boot project based on jdk 8, the version of skywalking is 7, please see my code snippet below.
@RequestMapping(value = "/testHeartBeat", method = RequestMethod.GET)
public RestHandlingResult testHeartBeat() {
    String traceId = TraceContext.traceId();
    HeartBeat heartBeat = new HeartBeat(800);
return RestHandlingResult.success(heartBeat);
the exception is shown below:
Caused by: java.lang.ClassNotFoundException: org.apache.skywalking.apm.toolkit.trace.TraceContext
    at ~[?:1.8.0_51]
    at java.lang.ClassLoader.loadClass( ~[?:1.8.0_51]
    at sun.misc.Launcher$AppClassLoader.loadClass( ~[?:1.8.0_51]
    at java.lang.ClassLoader.loadClass( ~[?:1.8.0_51]
    at org.springframework.boot.devtools.restart.classloader.RestartClassLoader.loadClass( ~[spring-boot-devtools-2.1.9.RELEASE.jar:2.1.9.RELEASE]
    at java.lang.ClassLoader.loadClass( ~[?:1.8.0_51]
    ... 83 more