Many-to-Many mapping is usually implemented a Join Table, suppose we have Type and Reason table and Reason table for many-to-many mapping. Every Type can have more than one Reasons and every Reasons is a part of more than one Type.

CREATE TABLE `rtrn_tp` (
	`rtrn_tp_cd` VARCHAR(20) NOT NULL DEFAULT '',
	`rtrn_tp_nm` VARCHAR(100) NULL DEFAULT NULL,
	`sort_num` INT(11) UNSIGNED NOT NULL DEFAULT '0',
	PRIMARY KEY (`rtrn_tp_cd`)
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB
;

CREATE TABLE `rtrn_rsn` (
	`rtrn_rsn_cd` VARCHAR(20) NOT NULL DEFAULT '',
	`rtrn_rsn_nm` VARCHAR(100) NULL DEFAULT NULL,
	`sort_num` INT(11) UNSIGNED NOT NULL DEFAULT '0',
	PRIMARY KEY (`rtrn_rsn_cd`)
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB
;

CREATE TABLE `rtrn_tp_rsn_cmbn` (
	`rtrn_tp_cd` VARCHAR(20) NOT NULL DEFAULT '',
	`rtrn_rsn_cd` VARCHAR(20) NOT NULL DEFAULT '',
	PRIMARY KEY (`rtrn_tp_cd`, `rtrn_rsn_cd`),
	INDEX `fk_rtrn_rsn` (`rtrn_rsn_cd`),
	CONSTRAINT `fk_rtrn_rsn` FOREIGN KEY (`rtrn_rsn_cd`) REFERENCES `rtrn_rsn` (`rtrn_rsn_cd`),
	CONSTRAINT `fk_rtrn_tp` FOREIGN KEY (`rtrn_tp_cd`) REFERENCES `rtrn_tp` (`rtrn_tp_cd`)
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB
;

There is a Join Table, RTRN_TP_RSN_CMBN, only have the relationship between Type (RTRN_TP) and Reason (RTRN_RSN)

package com.ns.spring.model;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Entity
@Table(name = "RTRN_TP")
public class RTRN_TP {

	@Id
	@Column(name = "rtrn_tp_cd")
	private String rtrnTpCd;

	@Column(name = "rtrn_tp_nm")
	private String rtrnTpNm;

	@Column(name = "sort_num")
	private int sortNum;

	@ManyToMany(targetEntity = RTRN_RSN.class, cascade = { CascadeType.ALL })
	@JoinTable(name = "RTRN_TP_RSN_CMBN", 
				joinColumns = { @JoinColumn(name = "rtrn_tp_cd") }, 
				inverseJoinColumns = { @JoinColumn(name = "rtrn_rsn_cd") })
	private Set<RTRN_RSN> rtrnRsns = new HashSet<RTRN_RSN>();

	public String getRtrnTpCd() {
		return rtrnTpCd;
	}

	public void setRtrnTpCd(String rtrnTpCd) {
		this.rtrnTpCd = rtrnTpCd;
	}

	public String getRtrnTpNm() {
		return rtrnTpNm;
	}

	public void setRtrnTpNm(String rtrnTpNm) {
		this.rtrnTpNm = rtrnTpNm;
	}

	public int getSortNum() {
		return sortNum;
	}

	public void setSortNum(int sortNum) {
		this.sortNum = sortNum;
	}

	public Set<RTRN_RSN> getRtrnRsns() {
		return rtrnRsns;
	}

	public void setRtrnRsns(Set<RTRN_RSN> rtrnRsns) {
		this.rtrnRsns = rtrnRsns;
	}

	public String toString() {
		return "Type Code:" + this.rtrnTpCd + ", Name:" + this.rtrnTpNm;
	}
}

package com.ns.spring.model;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name = "RTRN_RSN")
public class RTRN_RSN {

	@Id
	@Column(name = "rtrn_rsn_cd")
	private String rtrnRsnCd;
	
	@Column(name = "rtrn_rsn_nm")	
	private String rtrnRsnNm;
	
	@Column(name = "sort_num")	
	private int sortNum;

	public String getRtrnRsnCd() {
		return rtrnRsnCd;
	}

	public void setRtrnRsnCd(String rtrnRsnCd) {
		this.rtrnRsnCd = rtrnRsnCd;
	}

	public String getRtrnRsnNm() {
		return rtrnRsnNm;
	}

	public void setRtrnRsnNm(String rtrnRsnNm) {
		this.rtrnRsnNm = rtrnRsnNm;
	}

	public int getSortNum() {
		return sortNum;
	}

	public void setSortNum(int sortNum) {
		this.sortNum = sortNum;
	}

	public String toString() {
		return "Reason Code:" + this.rtrnRsnCd + ", Name:" + this.rtrnRsnNm;
	}
}

package com.ns.spring.model;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name = "RTRN_TP_RSN_CMBN")
public class RTRN_TP_RSN_CMBN implements Serializable {

	private static final long serialVersionUID = 4677612728077605784L;

	@Id
	@Column(name = "rtrn_tp_cd")
	private String rtrnTpCd;
	
	@Id
	@Column(name = "rtrn_rsn_cd")
	private String rtrnRsnCd;
	
	public String getRtrnTpCd() {
		return rtrnTpCd;
	}

	public void setRtrnTpCd(String rtrnTpCd) {
		this.rtrnTpCd = rtrnTpCd;
	}

	public String getRtrnRsnCd() {
		return rtrnRsnCd;
	}

	public void setRtrnRsnCd(String rtrnRsnCd) {
		this.rtrnRsnCd = rtrnRsnCd;
	}
	
	public String toString() {
		return "Type" + this.rtrnTpCd + ", Reason:" + this.rtrnRsnCd;
	}
}

@RequestMapping(value = "/cUrlValAttrbSubTpRsn01Jsp", params = "toBoundRsnList", method = RequestMethod.POST)
public ModelAndView toBoundRsnList(@ModelAttribute("RtrnTpModel") RtrnTpModel tp, ModelMap model, HttpServletRequest req) {

	String tpSelected = (String) req.getSession().getAttribute(CONST.TP_SELECTED.getVal());
	if (CommonBL.hasValue(tpSelected)) {
		RTRN_TP tpObj = this.tpSvc.findById(tpSelected);
		if (tpObj != null) {
			List<String> selectedCheckBox = TpRsnBL.getRsnToBound(tp, req);
			if (!CommonBL.isEmpty(selectedCheckBox)) {
				Set<RTRN_RSN> rtrnRsns = new HashSet<RTRN_RSN>();
				for (String code : selectedCheckBox) {
					RTRN_RSN rsnObj = this.rsnSvc.findById(code);
					if (rsnObj != null) {
						rtrnRsns.add(rsnObj);
						tpObj.setRtrnRsns(rtrnRsns);
						this.tpSvc.saveOrUpdate(tpObj);
					}
				}
			}
		}
	}
	return getMVSubTpRsn01(model, req);
}

This is how to use Many to Many in Spring MVC.