How to use the @aws-cdk/aws-codebuild.BuildSpec function in @aws-cdk/aws-codebuild

coffeeShopBucket.grantPut(buildRole);
        coffeeShopBucket.grantRead(buildRole);
        coffeeShopBucket.grantReadWrite(buildRole);
        coffeeShopBucket.grantWrite(buildRole);

        new codebuild.Project(this, 'CodeBuildProject', {
            role: buildRole,
            source: defaultSource,
            // Enable Docker AND custom caching
            cache: codebuild.Cache.local(codebuild.LocalCacheMode.DOCKER_LAYER, codebuild.LocalCacheMode.CUSTOM),
            environment: {
                buildImage: codebuild.LinuxBuildImage.AMAZON_LINUX_2,
                privileged: true,
            },
            buildSpec: codebuild.BuildSpec.fromObject({
                version: '0.2',
                phases: {
                    install:{
                        'runtime-versions': {
                            java: 'corretto8'
                        }
                    },
                    build: {
                        commands: [
                            'echo "Build all modules"',
                            'echo "Run Maven clean install to have all the required jars in local .m2 repository"',
                            'cd sources/coffeeshop',
                            'mvn clean install -Dmaven.test.skip=true'
                        ]
                    },
                    post_build: {