mysql报错：Specified key was too long; max key length is 767 bytes

package hello;

import java.util.Date;

public class User {
private String id;
private String name;
private String password;
private Date createTime;
private Date expireTime;

public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public Date getCreateTime() {
return createTime;
}
public void setCreateTime(Date createTime) {
this.createTime = createTime;
}
public Date getExpireTime() {
return expireTime;
}
public void setExpireTime(Date expireTime) {
this.expireTime = expireTime;
}
}

package hello;

import org.hibernate.cfg.Configuration;
import org.hibernate.tool.hbm2ddl.SchemaExport;

public class ExportDB {

/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
Configuration cfg = new Configuration().configure();
SchemaExport export = new SchemaExport(cfg);
export.create(true, true);
}

}