Multi-tenancy for SaaS applications

package com.example.demo.multitenancy;

import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;
import org.springframework.stereotype.Component;
import org.springframework.web.servlet.HandlerInterceptor;

@Component
public class TenantInterceptor implements HandlerInterceptor {

    private static final String TENANT_HEADER = "X-Tenant-ID";

    @Override
    public boolean preHandle(HttpServletRequest request,
                           HttpServletResponse response,
                           Object handler) {
        String tenantId = request.getHeader(TENANT_HEADER);

        if (tenantId == null) {
            // Alternatively, extract from subdomain
            String host = request.getServerName();
            tenantId = extractTenantFromHost(host);
        }

        if (tenantId != null) {
            TenantContext.setTenantId(tenantId);
        } else {
            response.setStatus(HttpServletResponse.SC_BAD_REQUEST);
            return false;
        }

        return true;
    }

    @Override
    public void afterCompletion(HttpServletRequest request,
                              HttpServletResponse response,
                              Object handler,
                              Exception ex) {
        TenantContext.clear();
    }

    private String extractTenantFromHost(String host) {
        // Extract tenant from subdomain: tenant.myapp.com
        if (host.contains(".")) {
            return host.split("\\.")[0];
        }
        return null;
    }
}

package com.example.demo.model;

import jakarta.persistence.*;
import org.hibernate.annotations.Filter;
import org.hibernate.annotations.FilterDef;
import org.hibernate.annotations.ParamDef;

@Entity
@Table(name = "products")
@FilterDef(
    name = "tenantFilter",
    parameters = @ParamDef(name = "tenantId", type = String.class)
)
@Filter(
    name = "tenantFilter",
    condition = "tenant_id = :tenantId"
)
public class Product {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "tenant_id", nullable = false)
    private String tenantId;

    private String name;
    private String description;
    private Double price;

    @PrePersist
    @PreUpdate
    public void setTenant() {
        this.tenantId = TenantContext.getTenantId();
    }

    // Getters and setters
}

package com.example.demo.config;

import com.example.demo.multitenancy.TenantContext;
import jakarta.persistence.EntityManager;
import org.hibernate.Session;
import org.springframework.stereotype.Component;

@Component
public class TenantFilter {

    private final EntityManager entityManager;

    public TenantFilter(EntityManager entityManager) {
        this.entityManager = entityManager;
    }

    public void enableFilter() {
        String tenantId = TenantContext.getTenantId();
        if (tenantId != null) {
            Session session = entityManager.unwrap(Session.class);
            session.enableFilter("tenantFilter")
                .setParameter("tenantId", tenantId);
        }
    }
}