这是一个问题，数据库的id生成方式GeneratedValue不设值报错

package com.bjsxt.hibernate;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name = "_teacher")
public class Teacher {
    private String id;
    private String name;
    private String title;

    @Id
    @GeneratedValue
  /*@GeneratedValue(generator = "system-unid")
    @GenericGenerator(name = "system-unid", strategy = "uuid")
    @Column(length = 32)*/
    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    @Column(name = "_name")
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

实体类，generatedValue不设值，数据库插入生成id报错

WARN org.hibernate.util.JDBCExceptionReporter:100 - SQL Error: 1364, SQLState: HY000
ERROR org.hibernate.util.JDBCExceptionReporter:101 - Field 'id' doesn't have a default value，

用第二种生成id的方式没有问题