diff --git a/scripts/raspberry-pi-display/README.md b/scripts/raspberry-pi-display/README.md
index 17310d2..2c3def3 100644
--- a/scripts/raspberry-pi-display/README.md
+++ b/scripts/raspberry-pi-display/README.md
@@ -87,6 +87,17 @@ sudo ./install-display.sh --skip-deps  # 依存の確認を省略する
 再実行しても設定は壊れません。systemd ユニットは内容が変わったときだけ書き換え、サービスが稼働中なら配置し直したコードを読ませるために再起動します。
 実行ユーザーが spi / gpio グループに入っていなければ追加します。グループ追加を反映するには一度ログインし直してください。
 
+## 更新
+
+導入後のコード更新はサーバーの更新に相乗りします。
+
+```bash
+sudo /opt/kidspos/update-app.sh
+```
+
+update-app.sh はリリースに同梱された表示サービスのコードで /opt/kidspos-display を差し替え、稼働中なら kidspos-display を再起動します。
+表示サービスだけを更新したい場合や systemd ユニット自体を変えたい場合は、リポジトリを取得し直して install-display.sh を実行してください。
+
 ## 起動と自動復旧
 
 install-display.sh を実行すると、配置・自動起動の有効化・起動までを一度に行います。
diff --git a/scripts/raspberry-pi/README.md b/scripts/raspberry-pi/README.md
index 44736f5..298707e 100644
--- a/scripts/raspberry-pi/README.md
+++ b/scripts/raspberry-pi/README.md
@@ -69,7 +69,10 @@ APK を確認せずサーバーだけ更新したい場合は --skip-apk を付
 実行中のスクリプトを壊さないよう、一時ディレクトリに展開してから差し替えるため、新しい内容は次回の実行から反映されます。
 差し替えたバージョンは .installed-scripts-version に記録され、失敗した場合は記録せず次回に再試行します。
 取得や展開に失敗しても警告のみでサーバーの更新は成功扱いです。スクリプトを差し替えたくない場合は --skip-self-update を付けてください。
-なお install.sh と systemd ユニットは daemon-reload を伴うため自動更新の対象外です。変更があった場合はリポジトリを取得し直して install.sh を実行してください。
+
+同じ配布物には e-Paper 表示サービスのコード（raspberry-pi-display）も含まれており、/opt/kidspos-display が存在すればそちらも同時に差し替え、稼働中なら kidspos-display を再起動します。
+表示サービスを導入していない Pi では何も行いません。差し替えたくない場合は --skip-display を付けてください。
+なお install.sh と install-display.sh と systemd ユニットは daemon-reload を伴うため自動更新の対象外です。変更があった場合はリポジトリを取得し直して各インストールスクリプトを実行してください。
 
 ### オフライン時（jar を持ち込む場合）
 
diff --git a/scripts/raspberry-pi/test-update-app.sh b/scripts/raspberry-pi/test-update-app.sh
index 8301f49..5260a38 100755
--- a/scripts/raspberry-pi/test-update-app.sh
+++ b/scripts/raspberry-pi/test-update-app.sh
@@ -7,6 +7,7 @@ UPDATE_SCRIPT="${SCRIPT_DIR}/update-app.sh"
 PASS_COUNT=0
 FAIL_COUNT=0
 CURRENT_TEST=""
+DISPLAY_MODULES="app.py config.py health.py layout.py renderer.py epaper.py"
 
 pass() {
     PASS_COUNT=$((PASS_COUNT + 1))
@@ -48,7 +49,8 @@ setup() {
     WORK=$(mktemp -d)
     APP_DIR="${WORK}/app"
     STUB_DIR="${WORK}/bin"
-    mkdir -p "$APP_DIR" "$STUB_DIR"
+    DISPLAY_DIR="${WORK}/display"
+    mkdir -p "$APP_DIR" "$STUB_DIR" "$DISPLAY_DIR"
 
     export CALL_LOG="${WORK}/calls.log"
     export HEALTH_OK_FILE="${WORK}/health-ok"
@@ -57,8 +59,17 @@ setup() {
     export SCRIPTS_TARBALL_FILE="${WORK}/scripts.tar.gz"
     export SCRIPTS_DOWNLOAD_EXIT=0
     export UPLOAD_APK_EXIT=0
+    export DISPLAY_INACTIVE_FILE="${WORK}/display-inactive"
+    export DISPLAY_RESTART_EXIT=0
     : > "$CALL_LOG"
 
+    DISPLAY_TARBALL_MODULES="$DISPLAY_MODULES"
+
+    local module
+    for module in $DISPLAY_MODULES; do
+        printf '# old-display-marker\n' > "${DISPLAY_DIR}/${module}"
+    done
+
     cat > "${STUB_DIR}/sudo" <<'EOF'
 #!/usr/bin/env bash
 exec "$@"
@@ -71,6 +82,12 @@ if [ "${1:-}" = "start" ] && [ -f "$FAIL_START_ONCE_FILE" ]; then
     rm -f "$FAIL_START_ONCE_FILE"
     exit 1
 fi
+if [ "${1:-}" = "is-active" ] && [ -f "$DISPLAY_INACTIVE_FILE" ]; then
+    exit 3
+fi
+if [ "${1:-}" = "restart" ]; then
+    exit "${DISPLAY_RESTART_EXIT:-0}"
+fi
 exit 0
 EOF
 
@@ -164,7 +181,41 @@ EOF
             printf '#!/usr/bin/env bash\n# new-script-marker\n' > "${src}/${name}"
         fi
     done
-    tar -czf "$SCRIPTS_TARBALL_FILE" -C "${WORK}/scripts-src" raspberry-pi
+
+    local entries=(raspberry-pi)
+    if [ -n "$DISPLAY_TARBALL_MODULES" ]; then
+        local display_src="${WORK}/scripts-src/raspberry-pi-display"
+        mkdir -p "$display_src"
+        local module
+        for module in $DISPLAY_TARBALL_MODULES; do
+            printf '# new-display-marker\n' > "${display_src}/${module}"
+        done
+        entries+=(raspberry-pi-display)
+    fi
+
+    tar -czf "$SCRIPTS_TARBALL_FILE" -C "${WORK}/scripts-src" "${entries[@]}"
+}
+
+assert_display_untouched() {
+    local module
+    for module in $DISPLAY_MODULES; do
+        if grep -q "new-display-marker" "${DISPLAY_DIR}/${module}" 2>/dev/null; then
+            fail_assert "$1"
+            return
+        fi
+    done
+    pass "$1"
+}
+
+assert_display_replaced() {
+    local module
+    for module in $DISPLAY_MODULES; do
+        if ! grep -q "new-display-marker" "${DISPLAY_DIR}/${module}" 2>/dev/null; then
+            fail_assert "$1 (未差し替え: ${module})"
+            return
+        fi
+    done
+    pass "$1"
 }
 
 assert_scripts_untouched() {
@@ -179,6 +230,7 @@ run_update() {
     set +e
     env PATH="${STUB_DIR}:${PATH}" \
         KIDSPOS_APP_DIR="$APP_DIR" \
+        KIDSPOS_DISPLAY_DIR="$DISPLAY_DIR" \
         KIDSPOS_HEALTH_RETRIES=2 \
         bash "$UPDATE_SCRIPT" "$@" > "${WORK}/out.log" 2>&1
     RC=$?
@@ -260,6 +312,7 @@ test_old_backups_are_pruned() {
     set +e
     env PATH="${STUB_DIR}:${PATH}" \
         KIDSPOS_APP_DIR="$APP_DIR" \
+        KIDSPOS_DISPLAY_DIR="$DISPLAY_DIR" \
         KIDSPOS_HEALTH_RETRIES=2 \
         KIDSPOS_BACKUP_KEEP=2 \
         bash "$UPDATE_SCRIPT" "${WORK}/new.jar" > "${WORK}/out.log" 2>&1
@@ -289,6 +342,7 @@ test_health_check_uses_timeout() {
     set +e
     env PATH="${STUB_DIR}:${PATH}" \
         KIDSPOS_APP_DIR="$APP_DIR" \
+        KIDSPOS_DISPLAY_DIR="$DISPLAY_DIR" \
         KIDSPOS_HEALTH_RETRIES=2 \
         KIDSPOS_HEALTH_TIMEOUT=3 \
         bash "$UPDATE_SCRIPT" "${WORK}/new.jar" > "${WORK}/out.log" 2>&1
@@ -478,7 +532,7 @@ test_self_update_when_jar_already_latest() {
     assert_eq 0 "$RC" "終了コードが 0"
     assert_contains "${WORK}/out.log" "すでに最新です" "最新である旨が出力される"
     assert_contains "${APP_DIR}/doctor.sh" "new-script-marker" "doctor.sh が差し替わる"
-    assert_not_contains "$CALL_LOG" "systemctl" "サービスは操作されない"
+    assert_not_contains "$CALL_LOG" "kidspos-server" "サーバーのサービスは操作されない"
     teardown
 }
 
@@ -601,6 +655,146 @@ test_self_update_not_run_when_rolled_back() {
     teardown
 }
 
+test_display_update_replaces_modules() {
+    setup "リリースに同梱された表示サービスのコードが差し替えられる"
+    touch "$HEALTH_OK_FILE"
+    write_release_json_with_scripts "v9.9.9"
+    make_scripts_tarball
+    run_update
+
+    assert_eq 0 "$RC" "終了コードが 0"
+    assert_display_replaced "表示サービスのコードが差し替わる"
+    assert_contains "${WORK}/out.log" "表示サービスを更新しました" "更新した旨が出力される"
+    assert_contains "$CALL_LOG" "systemctl restart kidspos-display" "表示サービスが再起動される"
+    assert_eq "v9.9.9" "$(cat "${APP_DIR}/.installed-scripts-version")" "スクリプトのバージョンが記録される"
+    if [ -x "${DISPLAY_DIR}/app.py" ]; then
+        pass "app.py に実行権限が付く"
+    else
+        fail_assert "app.py に実行権限が付く"
+    fi
+    teardown
+}
+
+test_display_restarts_only_when_active() {
+    setup "表示サービスが停止中なら再起動しない"
+    touch "$HEALTH_OK_FILE"
+    touch "$DISPLAY_INACTIVE_FILE"
+    write_release_json_with_scripts "v9.9.9"
+    make_scripts_tarball
+    run_update
+
+    assert_eq 0 "$RC" "終了コードが 0"
+    assert_display_replaced "表示サービスのコードが差し替わる"
+    assert_contains "${WORK}/out.log" "表示サービスは停止中のため再起動しません" "停止中である旨が出力される"
+    assert_not_contains "$CALL_LOG" "systemctl restart kidspos-display" "再起動は呼ばれない"
+    assert_eq "v9.9.9" "$(cat "${APP_DIR}/.installed-scripts-version")" "スクリプトのバージョンが記録される"
+    teardown
+}
+
+test_display_skipped_when_dir_missing() {
+    setup "表示サービスが導入されていなければ何もしない"
+    touch "$HEALTH_OK_FILE"
+    rm -rf "$DISPLAY_DIR"
+    write_release_json_with_scripts "v9.9.9"
+    make_scripts_tarball
+    run_update
+
+    assert_eq 0 "$RC" "終了コードが 0"
+    assert_contains "${WORK}/out.log" "表示サービスが導入されていないため更新しません" "スキップした旨が出力される"
+    assert_not_contains "$CALL_LOG" "systemctl restart kidspos-display" "再起動は呼ばれない"
+    assert_eq "v9.9.9" "$(cat "${APP_DIR}/.installed-scripts-version")" "スクリプトのバージョンが記録される"
+    if [ -d "$DISPLAY_DIR" ]; then
+        fail_assert "配置先は作成されない"
+    else
+        pass "配置先は作成されない"
+    fi
+    teardown
+}
+
+test_display_skipped_with_flag() {
+    setup "--skip-display を付けると表示サービスは差し替えられない"
+    touch "$HEALTH_OK_FILE"
+    write_release_json_with_scripts "v9.9.9"
+    make_scripts_tarball
+    run_update --skip-display
+
+    assert_eq 0 "$RC" "終了コードが 0"
+    assert_display_untouched "表示サービスのコードは差し替わらない"
+    assert_contains "${APP_DIR}/doctor.sh" "new-script-marker" "スクリプトは差し替わる"
+    assert_not_contains "$CALL_LOG" "systemctl restart kidspos-display" "再起動は呼ばれない"
+    assert_eq "v9.9.9" "$(cat "${APP_DIR}/.installed-scripts-version")" "スクリプトのバージョンが記録される"
+    teardown
+}
+
+test_display_partial_tarball_is_not_recorded() {
+    setup "配布物に足りない表示モジュールがあればバージョンを記録しない"
+    touch "$HEALTH_OK_FILE"
+    write_release_json_with_scripts "v9.9.9"
+    DISPLAY_TARBALL_MODULES="app.py config.py health.py renderer.py epaper.py"
+    make_scripts_tarball
+    run_update
+
+    assert_eq 0 "$RC" "終了コードが 0"
+    assert_contains "${WORK}/out.log" "配布物に含まれていません: raspberry-pi-display/layout.py" "不足が警告される"
+    assert_contains "${DISPLAY_DIR}/app.py" "new-display-marker" "含まれている分は差し替わる"
+    if [ -e "${APP_DIR}/.installed-scripts-version" ]; then
+        fail_assert "スクリプトのバージョンは記録されない"
+    else
+        pass "スクリプトのバージョンは記録されない"
+    fi
+    teardown
+}
+
+test_display_missing_in_tarball() {
+    setup "配布物に表示サービスが無ければバージョンを記録しない"
+    touch "$HEALTH_OK_FILE"
+    write_release_json_with_scripts "v9.9.9"
+    DISPLAY_TARBALL_MODULES=""
+    make_scripts_tarball
+    run_update
+
+    assert_eq 0 "$RC" "終了コードが 0"
+    assert_contains "${WORK}/out.log" "配布物に raspberry-pi-display が含まれていません" "不足が警告される"
+    assert_contains "${WORK}/out.log" "更新が完了しました" "サーバーの更新は完了扱いになる"
+    assert_display_untouched "表示サービスのコードは差し替わらない"
+    if [ -e "${APP_DIR}/.installed-scripts-version" ]; then
+        fail_assert "スクリプトのバージョンは記録されない"
+    else
+        pass "スクリプトのバージョンは記録されない"
+    fi
+    teardown
+}
+
+test_display_restart_failure_does_not_fail_update() {
+    setup "表示サービスの再起動に失敗してもサーバーの更新は成功扱いになる"
+    touch "$HEALTH_OK_FILE"
+    write_release_json_with_scripts "v9.9.9"
+    make_scripts_tarball
+    export DISPLAY_RESTART_EXIT=1
+    run_update
+
+    assert_eq 0 "$RC" "終了コードが 0"
+    assert_contains "${WORK}/out.log" "表示サービスの再起動に失敗しました" "警告が出力される"
+    assert_contains "${WORK}/out.log" "更新が完了しました" "サーバーの更新は完了扱いになる"
+    if [ -e "${APP_DIR}/.installed-scripts-version" ]; then
+        fail_assert "スクリプトのバージョンは記録されない"
+    else
+        pass "スクリプトのバージョンは記録されない"
+    fi
+    teardown
+}
+
+test_display_not_updated_for_local_jar() {
+    setup "持ち込んだ jar での更新では表示サービスは差し替えられない"
+    touch "$HEALTH_OK_FILE"
+    run_update "${WORK}/new.jar"
+
+    assert_eq 0 "$RC" "終了コードが 0"
+    assert_display_untouched "表示サービスのコードは差し替わらない"
+    assert_not_contains "$CALL_LOG" "systemctl restart kidspos-display" "再起動は呼ばれない"
+    teardown
+}
+
 test_success_path
 test_health_check_failure_rolls_back
 test_start_failure_rolls_back
@@ -628,6 +822,14 @@ test_self_update_extract_failure_does_not_fail_update
 test_self_update_partial_tarball_is_not_recorded
 test_self_update_skipped_for_local_jar
 test_self_update_not_run_when_rolled_back
+test_display_update_replaces_modules
+test_display_restarts_only_when_active
+test_display_skipped_when_dir_missing
+test_display_skipped_with_flag
+test_display_partial_tarball_is_not_recorded
+test_display_missing_in_tarball
+test_display_restart_failure_does_not_fail_update
+test_display_not_updated_for_local_jar
 
 echo ""
 echo "passed: $PASS_COUNT, failed: $FAIL_COUNT"
diff --git a/scripts/raspberry-pi/update-app.sh b/scripts/raspberry-pi/update-app.sh
index 2be675f..2633b73 100755
--- a/scripts/raspberry-pi/update-app.sh
+++ b/scripts/raspberry-pi/update-app.sh
@@ -13,6 +13,9 @@ SERVER_URL="${KIDSPOS_SERVER_URL:-http://localhost:8080}"
 ASSET_NAME="app.jar"
 SCRIPTS_ASSET="kidspos-scripts.tar.gz"
 MANAGED_SCRIPTS="update-app.sh doctor.sh upload-apk.sh"
+DISPLAY_DIR="${KIDSPOS_DISPLAY_DIR:-/opt/kidspos-display}"
+DISPLAY_SERVICE="${KIDSPOS_DISPLAY_SERVICE:-kidspos-display}"
+DISPLAY_FILES="app.py config.py health.py layout.py renderer.py epaper.py"
 
 JAR_PATH="${APP_DIR}/${JAR_NAME}"
 DB_PATH="${APP_DIR}/kidspos.db"
@@ -29,6 +32,7 @@ usage() {
     echo "  sudo $0 --force            同一バージョンでも強制的に再インストール"
     echo "  sudo $0 --skip-apk         サーバーの更新のみ行い、APK の確認は行わない"
     echo "  sudo $0 --skip-self-update スクリプト自身の更新は行わない"
+    echo "  sudo $0 --skip-display     e-Paper 表示サービスの更新は行わない"
     exit 1
 }
 
@@ -38,6 +42,7 @@ fail() { echo "[update-app] ERROR: $*" >&2; exit 1; }
 FORCE=false
 SKIP_APK=false
 SKIP_SELF_UPDATE=false
+SKIP_DISPLAY=false
 LOCAL_JAR=""
 SCRIPTS_URL=""
 for arg in "$@"; do
@@ -45,6 +50,7 @@ for arg in "$@"; do
         --force) FORCE=true ;;
         --skip-apk) SKIP_APK=true ;;
         --skip-self-update) SKIP_SELF_UPDATE=true ;;
+        --skip-display) SKIP_DISPLAY=true ;;
         -h|--help) usage ;;
         -*)
             echo "[update-app] 不明なオプション: $arg" >&2
@@ -54,6 +60,63 @@ for arg in "$@"; do
     esac
 done
 
+# 表示サービスを導入していない Pi もあるため、配置先が無ければ何もしない。
+# 差し替えに失敗した場合は呼び出し元でバージョンを記録させず、次回の更新で再試行させる
+update_display() {
+    if [ "$SKIP_DISPLAY" = true ]; then
+        return 0
+    fi
+    if [ ! -d "$DISPLAY_DIR" ]; then
+        log "表示サービスが導入されていないため更新しません: $DISPLAY_DIR"
+        return 0
+    fi
+
+    local staged="${STAGE_DIR}/raspberry-pi-display"
+    if [ ! -d "$staged" ]; then
+        log "WARN: 配布物に raspberry-pi-display が含まれていません"
+        return 1
+    fi
+
+    local ok=true
+    local module src
+    for module in $DISPLAY_FILES; do
+        src="${staged}/${module}"
+        if [ ! -f "$src" ]; then
+            log "WARN: 配布物に含まれていません: raspberry-pi-display/${module}"
+            ok=false
+            continue
+        fi
+        if ! mv -f "$src" "${DISPLAY_DIR}/${module}"; then
+            log "WARN: 差し替えに失敗しました: ${DISPLAY_DIR}/${module}"
+            ok=false
+            continue
+        fi
+        # install-display.sh がサービス実行ユーザーに所有者を合わせているため、
+        # root で置き直したファイルも配置先ディレクトリと同じ所有者に戻す
+        if ! chown --reference="$DISPLAY_DIR" "${DISPLAY_DIR}/${module}"; then
+            log "WARN: 所有者を合わせられませんでした: ${DISPLAY_DIR}/${module}"
+        fi
+    done
+    if [ -f "${DISPLAY_DIR}/app.py" ]; then
+        chmod +x "${DISPLAY_DIR}/app.py"
+    fi
+
+    if [ "$ok" != true ]; then
+        return 1
+    fi
+
+    log "表示サービスを更新しました: $DISPLAY_DIR"
+    if systemctl is-active --quiet "$DISPLAY_SERVICE"; then
+        if ! sudo systemctl restart "$DISPLAY_SERVICE"; then
+            log "WARN: 表示サービスの再起動に失敗しました: $DISPLAY_SERVICE"
+            return 1
+        fi
+    else
+        log "表示サービスは停止中のため再起動しません: $DISPLAY_SERVICE"
+    fi
+    return 0
+}
+
 # スクリプトの更新に失敗してもサーバーの更新は完了しているため、警告のみで成功扱いにする。
 # 成功した分だけ差し替え、全て成功したときだけバージョンを記録して次回の再試行に備える
 self_update() {
@@ -109,6 +172,9 @@ self_update() {
             all_ok=false
         fi
     done
+    if ! update_display; then
+        all_ok=false
+    fi
     rm -rf "$STAGE_DIR"
 
     if [ "$all_ok" = true ]; then
diff --git a/src/main/kotlin/info/nukoneko/kidspos/receipt/ReceiptPrinter.kt b/src/main/kotlin/info/nukoneko/kidspos/receipt/ReceiptPrinter.kt
index 1ba91b9..2f60b88 100644
--- a/src/main/kotlin/info/nukoneko/kidspos/receipt/ReceiptPrinter.kt
+++ b/src/main/kotlin/info/nukoneko/kidspos/receipt/ReceiptPrinter.kt
@@ -4,6 +4,7 @@ import info.nukoneko.kidspos.common.PrintCommand
 import info.nukoneko.kidspos.common.toAllEm
 import info.nukoneko.kidspos.common.toEm
 import java.io.IOException
+import java.net.InetSocketAddress
 import java.net.Socket
 import java.nio.charset.Charset
 import java.text.SimpleDateFormat
@@ -85,16 +86,21 @@ class ReceiptPrinter(
         command.newLine()
     }
 
+    // 到達できないプリンターへの接続は OS 既定では数分待たされるため、必ず上限を設ける
     @Throws(IOException::class)
-    fun print() {
-        Socket(ipOrHost, port).use { socket ->
+    fun print(timeoutMillis: Int = DEFAULT_TIMEOUT_MILLIS) {
+        Socket().use { socket ->
+            socket.soTimeout = timeoutMillis
+            socket.connect(InetSocketAddress(ipOrHost, port), timeoutMillis)
             socket.getOutputStream().use {
                 it.write(command.build())
+                it.flush()
             }
         }
     }
 
     companion object {
+        const val DEFAULT_TIMEOUT_MILLIS = 3_000
         private const val MAX_ROW_TEXT_NUM = 20
         private val dateFormat =
             SimpleDateFormat("yyyy年MM月dd日(E) HH時mm分ss秒")
diff --git a/src/main/kotlin/info/nukoneko/kidspos/server/config/AppProperties.kt b/src/main/kotlin/info/nukoneko/kidspos/server/config/AppProperties.kt
index 96e5a6a..445f45a 100644
--- a/src/main/kotlin/info/nukoneko/kidspos/server/config/AppProperties.kt
+++ b/src/main/kotlin/info/nukoneko/kidspos/server/config/AppProperties.kt
@@ -15,6 +15,7 @@ data class AppProperties(
             val host: String = "localhost",
             val port: Int = 9100,
             val statusCacheSeconds: Int = 15,
+            val connectTimeoutMillis: Int = 3_000,
         )
     }
 
diff --git a/src/main/kotlin/info/nukoneko/kidspos/server/controller/api/SaleApiController.kt b/src/main/kotlin/info/nukoneko/kidspos/server/controller/api/SaleApiController.kt
index a02dc14..b1f9347 100644
--- a/src/main/kotlin/info/nukoneko/kidspos/server/controller/api/SaleApiController.kt
+++ b/src/main/kotlin/info/nukoneko/kidspos/server/controller/api/SaleApiController.kt
@@ -51,7 +51,7 @@ class SaleApiController(
             when (val result = saleProcessingService.processSaleWithValidation(saleBean, items)) {
                 is SaleResult.Success -> {
                     // Print receipt
-                    receiptService.printReceipt(
+                    receiptService.printReceiptAsync(
                         request.storeId,
                         items,
                         request.deposit,
@@ -106,7 +106,7 @@ class SaleApiController(
             when (val result = saleProcessingService.processSaleWithValidation(saleBean, items)) {
                 is SaleResult.Success -> {
                     // Print receipt
-                    receiptService.printReceipt(
+                    receiptService.printReceiptAsync(
                         saleBean.storeId,
                         items,
                         saleBean.deposit,
diff --git a/src/main/kotlin/info/nukoneko/kidspos/server/controller/front/ApkController.kt b/src/main/kotlin/info/nukoneko/kidspos/server/controller/front/ApkController.kt
index 92a08a4..27baa81 100644
--- a/src/main/kotlin/info/nukoneko/kidspos/server/controller/front/ApkController.kt
+++ b/src/main/kotlin/info/nukoneko/kidspos/server/controller/front/ApkController.kt
@@ -25,6 +25,7 @@ class ApkController(
 
         model.addAttribute("versions", versions)
         model.addAttribute("latestVersion", latestVersion)
+        model.addAttribute("versionOrderConflicts", apkVersionService.detectVersionOrderConflicts(versions))
         model.addAttribute("title", "APKバージョン管理")
 
         return "apk/index"
diff --git a/src/main/kotlin/info/nukoneko/kidspos/server/service/ApkVersionService.kt b/src/main/kotlin/info/nukoneko/kidspos/server/service/ApkVersionService.kt
index d692614..77b572b 100644
--- a/src/main/kotlin/info/nukoneko/kidspos/server/service/ApkVersionService.kt
+++ b/src/main/kotlin/info/nukoneko/kidspos/server/service/ApkVersionService.kt
@@ -17,6 +17,14 @@ import java.nio.file.Paths
 import java.nio.file.StandardCopyOption
 import java.time.LocalDateTime
 
+/**
+ * バージョン名の新しさとバージョンコードの大小が食い違う組み合わせ
+ */
+data class VersionOrderConflict(
+    val newerName: ApkVersionEntity,
+    val higherCode: ApkVersionEntity,
+)
+
 @Service
 @Transactional
 class ApkVersionService(
@@ -142,6 +150,59 @@ class ApkVersionService(
     @Transactional(readOnly = true)
     fun getAllVersions(): List<ApkVersionEntity> = apkVersionRepository.findAllByOrderByVersionCodeDesc()
 
+    /**
+     * バージョン名の新しさとバージョンコードの大小が食い違う組み合わせを洗い出す
+     *
+     * 最新バージョンはバージョンコードの降順で決まるため、
+     * 手入力ミスで大きすぎるコードが登録されると古いAPKが配信され続ける。
+     */
+    fun detectVersionOrderConflicts(versions: List<ApkVersionEntity>): List<VersionOrderConflict> =
+        versions.indices.flatMap { left ->
+            (left + 1 until versions.size).mapNotNull { right ->
+                conflictBetween(versions[left], versions[right])
+            }
+        }
+
+    private fun conflictBetween(
+        left: ApkVersionEntity,
+        right: ApkVersionEntity,
+    ): VersionOrderConflict? {
+        val nameOrder = compareVersionNames(left.version, right.version)
+        val codeOrder = left.versionCode.compareTo(right.versionCode)
+        if (nameOrder == 0 || codeOrder == 0 || (nameOrder > 0) == (codeOrder > 0)) {
+            return null
+        }
+        return if (nameOrder > 0) {
+            VersionOrderConflict(newerName = left, higherCode = right)
+        } else {
+            VersionOrderConflict(newerName = right, higherCode = left)
+        }
+    }
+
+    private fun compareVersionNames(
+        left: String,
+        right: String,
+    ): Int {
+        val leftParts = left.split('.')
+        val rightParts = right.split('.')
+        for (index in 0 until maxOf(leftParts.size, rightParts.size)) {
+            val leftPart = leftParts.getOrElse(index) { "" }
+            val rightPart = rightParts.getOrElse(index) { "" }
+            val leftNumber = leftPart.toIntOrNull()
+            val rightNumber = rightPart.toIntOrNull()
+            val order =
+                if (leftNumber != null && rightNumber != null) {
+                    leftNumber.compareTo(rightNumber)
+                } else {
+                    leftPart.compareTo(rightPart)
+                }
+            if (order != 0) {
+                return order
+            }
+        }
+        return 0
+    }
+
     @Transactional(readOnly = true)
     fun getVersionById(id: Long): ApkVersionEntity =
         apkVersionRepository
diff --git a/src/main/kotlin/info/nukoneko/kidspos/server/service/ReceiptService.kt b/src/main/kotlin/info/nukoneko/kidspos/server/service/ReceiptService.kt
index 541ca40..fcdcdda 100644
--- a/src/main/kotlin/info/nukoneko/kidspos/server/service/ReceiptService.kt
+++ b/src/main/kotlin/info/nukoneko/kidspos/server/service/ReceiptService.kt
@@ -5,10 +5,15 @@ import info.nukoneko.kidspos.receipt.ReceiptPrinter
 import info.nukoneko.kidspos.server.config.AppProperties
 import info.nukoneko.kidspos.server.controller.dto.request.ItemBean
 import info.nukoneko.kidspos.server.entity.ItemEntity
+import jakarta.annotation.PreDestroy
 import org.slf4j.LoggerFactory
 import org.springframework.stereotype.Service
 import java.io.IOException
 import java.util.*
+import java.util.concurrent.LinkedBlockingQueue
+import java.util.concurrent.RejectedExecutionException
+import java.util.concurrent.ThreadPoolExecutor
+import java.util.concurrent.TimeUnit
 
 /**
  * Service responsible for receipt operations
@@ -21,29 +26,61 @@ class ReceiptService(
 ) {
     private val logger = LoggerFactory.getLogger(ReceiptService::class.java)
 
+    private val executor =
+        ThreadPoolExecutor(
+            PRINT_THREADS,
+            PRINT_THREADS,
+            KEEP_ALIVE_SECONDS,
+            TimeUnit.SECONDS,
+            LinkedBlockingQueue(QUEUE_CAPACITY),
+            { runnable -> Thread(runnable, "receipt-printer").apply { isDaemon = true } },
+        ).apply { allowCoreThreadTimeOut(true) }
+
     /**
-     * Print receipt for a sale
+     * レシート印刷を依頼する
+     *
+     * 到達できないプリンターは接続タイムアウトまで待たされるため、
+     * 送信は別スレッドに任せて会計のレスポンスを待たせない。
+     * 戻り値は印刷を実際に依頼できたかどうかで、印刷の成否ではない。
      */
-    fun printReceipt(
+    fun printReceiptAsync(
         storeId: Int,
         items: List<ItemBean>,
         deposit: Int,
     ): Boolean {
         logger.debug("Printing receipt for store: {}, items: {}", storeId, items.size)
 
-        return try {
-            val receiptDetail = createReceiptDetail(storeId, items, deposit)
-            val printerIp = getPrinterIp(storeId) ?: return false
+        val printerIp = getPrinterIp(storeId) ?: return false
 
-            sendToPrinter(printerIp, receiptDetail)
-            logger.info("Receipt printed successfully for store: {}", storeId)
+        val receiptDetail =
+            try {
+                createReceiptDetail(storeId, items, deposit)
+            } catch (e: Exception) {
+                logger.error("Failed to build receipt for store: {}", storeId, e)
+                return false
+            }
+
+        return try {
+            executor.execute {
+                try {
+                    sendToPrinter(printerIp, receiptDetail)
+                    logger.info("Receipt printed successfully for store: {}", storeId)
+                } catch (e: Exception) {
+                    logger.error("Failed to print receipt for store: {}", storeId, e)
+                }
+            }
             true
-        } catch (e: Exception) {
-            logger.error("Failed to print receipt for store: {}", storeId, e)
+        } catch (e: RejectedExecutionException) {
+            logger.error("Print queue is full, receipt discarded for store: {}", storeId, e)
             false
         }
     }
 
+    @PreDestroy
+    fun shutdown() {
+        executor.shutdownNow()
+    }
+
     /**
      * Create receipt detail object
      */
@@ -99,15 +136,16 @@ class ReceiptService(
         printerIp: String,
         receiptDetail: ReceiptDetail,
     ) {
+        val printerProperties = appProperties.receipt.printer
         val printer =
             ReceiptPrinter(
                 printerIp,
-                appProperties.receipt.printer.port,
+                printerProperties.port,
                 receiptDetail,
             )
 
         try {
-            printer.print()
+            printer.print(printerProperties.connectTimeoutMillis)
             logger.debug("Receipt sent to printer at: {}", printerIp)
         } catch (e: IOException) {
             logger.error("Failed to send receipt to printer at {}: {}", printerIp, e.message, e)
@@ -147,4 +185,10 @@ class ReceiptService(
      * Validate printer configuration for store
      */
     fun validatePrinterConfiguration(storeId: Int): Boolean = getPrinterIp(storeId) != null
+
+    companion object {
+        const val PRINT_THREADS = 2
+        const val KEEP_ALIVE_SECONDS = 60L
+        const val QUEUE_CAPACITY = 64
+    }
 }
diff --git a/src/main/resources/application.yaml b/src/main/resources/application.yaml
index c216cbd..847d730 100644
--- a/src/main/resources/application.yaml
+++ b/src/main/resources/application.yaml
@@ -50,6 +50,7 @@ app:
       host: ${RECEIPT_PRINTER_HOST:localhost}
       port: ${RECEIPT_PRINTER_PORT:9100}
       status-cache-seconds: ${RECEIPT_PRINTER_STATUS_CACHE_SECONDS:15}
+      connect-timeout-millis: ${RECEIPT_PRINTER_CONNECT_TIMEOUT_MILLIS:3000}
   barcode:
     qr-size: ${QR_CODE_SIZE:200}
     pdf:
diff --git a/src/main/resources/templates/apk/index.html b/src/main/resources/templates/apk/index.html
index a8d3034..913bbb6 100644
--- a/src/main/resources/templates/apk/index.html
+++ b/src/main/resources/templates/apk/index.html
@@ -31,6 +31,21 @@ <h1 class="h4 mb-0">
                         <button type="button" class="btn-close" data-bs-dismiss="alert"></button>
                     </div>
 
+                    <div th:if="${not #lists.isEmpty(versionOrderConflicts)}" class="alert alert-warning">
+                        <h5 class="alert-heading">バージョン番号の並びが逆転しています</h5>
+                        <p>最新バージョンはバージョンコードの大きい順に決まります。次の組み合わせはバージョン名の新しさと逆になっているため、アプリに古いAPKが配信されます。誤って登録されたバージョンコードを削除してください。</p>
+                        <ul class="mb-0">
+                            <li th:each="conflict : ${versionOrderConflicts}">
+                                <strong th:text="${conflict.newerName.version}"></strong>
+                                (コード <span th:text="${conflict.newerName.versionCode}"></span>)
+                                のほうが新しいバージョン名ですが、
+                                <strong th:text="${conflict.higherCode.version}"></strong>
+                                (コード <span th:text="${conflict.higherCode.versionCode}"></span>)
+                                のバージョンコードが大きいため最新として扱われます。
+                            </li>
+                        </ul>
+                    </div>
+
                     <div th:if="${latestVersion}" class="alert alert-info">
                         <h5 class="alert-heading">最新バージョン</h5>
                         <p class="mb-0">
diff --git a/src/test/kotlin/info/nukoneko/kidspos/receipt/ReceiptPrinterTest.kt b/src/test/kotlin/info/nukoneko/kidspos/receipt/ReceiptPrinterTest.kt
new file mode 100644
index 0000000..3d732dc
--- /dev/null
+++ b/src/test/kotlin/info/nukoneko/kidspos/receipt/ReceiptPrinterTest.kt
@@ -0,0 +1,78 @@
+package info.nukoneko.kidspos.receipt
+
+import info.nukoneko.kidspos.server.entity.ItemEntity
+import org.junit.jupiter.api.Assertions.assertThrows
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.Test
+import java.io.IOException
+import java.net.ServerSocket
+import java.util.Date
+import java.util.concurrent.ArrayBlockingQueue
+import java.util.concurrent.TimeUnit
+
+class ReceiptPrinterTest {
+    private fun detail() =
+        ReceiptDetail(
+            items = listOf(ItemEntity(id = 1, barcode = "0123456789", name = "あめ", price = 100)),
+            storeName = "テスト店",
+            deposit = 500,
+            transactionId = "0123456789",
+            createdAt = Date(),
+        )
+
+    @Test
+    fun `プリンターへ印字データを送信する`() {
+        val received = ArrayBlockingQueue<ByteArray>(1)
+        ServerSocket(0).use { server ->
+            Thread {
+                server.accept().use { socket ->
+                    received.put(socket.getInputStream().readBytes())
+                }
+            }.apply { isDaemon = true }.start()
+
+            ReceiptPrinter("127.0.0.1", server.localPort, detail()).print()
+
+            val bytes = received.poll(WAIT_LIMIT_MILLIS, TimeUnit.MILLISECONDS)
+            assertTrue(bytes != null && bytes.isNotEmpty(), "印字データが送信される")
+        }
+    }
+
+    @Test
+    fun `接続を拒否されたら待たずに失敗する`() {
+        val closedPort = ServerSocket(0).use { it.localPort }
+
+        val elapsedMillis =
+            measureMillis {
+                assertThrows(IOException::class.java) {
+                    ReceiptPrinter("127.0.0.1", closedPort, detail()).print(TIMEOUT_MILLIS)
+                }
+            }
+
+        assertTrue(elapsedMillis < WAIT_LIMIT_MILLIS, "すぐに失敗する: ${elapsedMillis}ms")
+    }
+
+    @Test
+    fun `到達できないプリンターは指定したタイムアウトで打ち切る`() {
+        val elapsedMillis =
+            measureMillis {
+                assertThrows(IOException::class.java) {
+                    ReceiptPrinter(UNROUTABLE_HOST, 9100, detail()).print(TIMEOUT_MILLIS)
+                }
+            }
+
+        assertTrue(elapsedMillis < WAIT_LIMIT_MILLIS, "指定したタイムアウトで打ち切られる: ${elapsedMillis}ms")
+    }
+
+    private fun measureMillis(block: () -> Unit): Long {
+        val startedAt = System.nanoTime()
+        block()
+        return TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startedAt)
+    }
+
+    companion object {
+        // RFC 5737 のドキュメント用アドレス。経路が無いため接続タイムアウトの経路を通る
+        private const val UNROUTABLE_HOST = "192.0.2.1"
+        private const val TIMEOUT_MILLIS = 300
+        private const val WAIT_LIMIT_MILLIS = 5_000L
+    }
+}
diff --git a/src/test/kotlin/info/nukoneko/kidspos/server/controller/api/SaleApiControllerTest.kt b/src/test/kotlin/info/nukoneko/kidspos/server/controller/api/SaleApiControllerTest.kt
index e51600b..cc75005 100644
--- a/src/test/kotlin/info/nukoneko/kidspos/server/controller/api/SaleApiControllerTest.kt
+++ b/src/test/kotlin/info/nukoneko/kidspos/server/controller/api/SaleApiControllerTest.kt
@@ -96,7 +96,7 @@ class SaleApiControllerTest {
         `when`(itemParsingService.parseItemsFromIds("1,2")).thenReturn(testItems)
         `when`(saleProcessingService.processSaleWithValidation(any(), any()))
             .thenReturn(SaleResult.Success(testSale, summary))
-        `when`(receiptService.printReceipt(any(), any(), any())).thenReturn(true)
+        `when`(receiptService.printReceiptAsync(any(), any(), any())).thenReturn(true)
 
         // When & Then
         mockMvc
@@ -139,7 +139,7 @@ class SaleApiControllerTest {
 
         verify(itemParsingService).parseItemsFromIds("1,2")
         verify(saleProcessingService).processSaleWithValidation(any(), any())
-        verify(receiptService, never()).printReceipt(any(), any(), any())
+        verify(receiptService, never()).printReceiptAsync(any(), any(), any())
     }
 
     @Test
diff --git a/src/test/kotlin/info/nukoneko/kidspos/server/controller/api/SaleApiControllerUnitTest.kt b/src/test/kotlin/info/nukoneko/kidspos/server/controller/api/SaleApiControllerUnitTest.kt
index 92c657d..6c51dad 100644
--- a/src/test/kotlin/info/nukoneko/kidspos/server/controller/api/SaleApiControllerUnitTest.kt
+++ b/src/test/kotlin/info/nukoneko/kidspos/server/controller/api/SaleApiControllerUnitTest.kt
@@ -87,7 +87,7 @@ class SaleApiControllerUnitTest {
         `when`(itemParsingService.parseItemsFromIds("1,2")).thenReturn(testItems)
         `when`(saleProcessingService.processSaleWithValidation(expectedSaleBean, testItems))
             .thenReturn(SaleResult.Success(testSale, summary))
-        `when`(receiptService.printReceipt(1, testItems, 400)).thenReturn(true)
+        `when`(receiptService.printReceiptAsync(1, testItems, 400)).thenReturn(true)
 
         // When
         val result = controller.createSale(request)
@@ -105,7 +105,7 @@ class SaleApiControllerUnitTest {
 
         verify(itemParsingService).parseItemsFromIds("1,2")
         verify(saleProcessingService).processSaleWithValidation(expectedSaleBean, testItems)
-        verify(receiptService).printReceipt(1, testItems, 400)
+        verify(receiptService).printReceiptAsync(1, testItems, 400)
     }
 
     @Test
diff --git a/src/test/kotlin/info/nukoneko/kidspos/server/controller/front/ApkControllerTest.kt b/src/test/kotlin/info/nukoneko/kidspos/server/controller/front/ApkControllerTest.kt
index b450cf8..24fd23a 100644
--- a/src/test/kotlin/info/nukoneko/kidspos/server/controller/front/ApkControllerTest.kt
+++ b/src/test/kotlin/info/nukoneko/kidspos/server/controller/front/ApkControllerTest.kt
@@ -4,6 +4,9 @@ import info.nukoneko.kidspos.server.domain.exception.InvalidFileException
 import info.nukoneko.kidspos.server.entity.ApkVersionEntity
 import info.nukoneko.kidspos.server.service.ApkManifestInfo
 import info.nukoneko.kidspos.server.service.ApkVersionService
+import info.nukoneko.kidspos.server.service.VersionOrderConflict
+import org.hamcrest.Matchers.containsString
+import org.hamcrest.Matchers.not
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.extension.ExtendWith
 import org.mockito.Mockito.*
@@ -34,6 +37,56 @@ class ApkControllerTest {
         contentType: String = "application/vnd.android.package-archive",
     ) = MockMultipartFile("file", name, contentType, ByteArray(100))
 
+    @Test
+    fun `GET index should expose detected version order conflicts`() {
+        val mistyped = apkVersion(id = 1L, version = "1.0.10", versionCode = 100)
+        val newest = apkVersion(id = 2L, version = "1.0.11", versionCode = 12)
+        val conflicts = listOf(VersionOrderConflict(newerName = newest, higherCode = mistyped))
+        val versions = listOf(mistyped, newest)
+
+        whenever(apkVersionService.getAllVersions()).thenReturn(versions)
+        whenever(apkVersionService.getLatestVersion()).thenReturn(mistyped)
+        whenever(apkVersionService.detectVersionOrderConflicts(versions)).thenReturn(conflicts)
+
+        mockMvc
+            .perform(get("/apk"))
+            .andExpect(status().isOk)
+            .andExpect(view().name("apk/index"))
+            .andExpect(model().attribute("versionOrderConflicts", conflicts))
+            .andExpect(content().string(containsString("バージョン番号の並びが逆転しています")))
+            .andExpect(content().string(containsString("1.0.11")))
+    }
+
+    @Test
+    fun `GET index should expose an empty conflict list when versions are consistent`() {
+        val versions = listOf(apkVersion(id = 1L, version = "1.0.11", versionCode = 12))
+
+        whenever(apkVersionService.getAllVersions()).thenReturn(versions)
+        whenever(apkVersionService.getLatestVersion()).thenReturn(versions.first())
+        whenever(apkVersionService.detectVersionOrderConflicts(versions)).thenReturn(emptyList())
+
+        mockMvc
+            .perform(get("/apk"))
+            .andExpect(status().isOk)
+            .andExpect(model().attribute("versionOrderConflicts", emptyList<VersionOrderConflict>()))
+            .andExpect(content().string(not(containsString("バージョン番号の並びが逆転しています"))))
+    }
+
+    private fun apkVersion(
+        id: Long,
+        version: String,
+        versionCode: Int,
+    ) = ApkVersionEntity(
+        id = id,
+        version = version,
+        versionCode = versionCode,
+        fileName = "kidspos-v$version.apk",
+        fileSize = 1000L,
+        filePath = "/uploads/apk/kidspos-v$version.apk",
+        isActive = true,
+        uploadedAt = LocalDateTime.now(),
+    )
+
     @Test
     fun `POST analyze should return version information as JSON`() {
         whenever(apkVersionService.analyzeApk(any())).thenReturn(ApkManifestInfo("1.2.3", 10203))
diff --git a/src/test/kotlin/info/nukoneko/kidspos/server/service/ApkVersionServiceTest.kt b/src/test/kotlin/info/nukoneko/kidspos/server/service/ApkVersionServiceTest.kt
index 62a99d2..a9eac6c 100644
--- a/src/test/kotlin/info/nukoneko/kidspos/server/service/ApkVersionServiceTest.kt
+++ b/src/test/kotlin/info/nukoneko/kidspos/server/service/ApkVersionServiceTest.kt
@@ -247,6 +247,84 @@ class ApkVersionServiceTest {
         assertNull(result)
     }
 
+    @Test
+    fun `detectVersionOrderConflicts should report a version code that contradicts the version name`() {
+        val mistyped = versionOf(id = 1L, version = "1.0.10", versionCode = 100)
+        val newest = versionOf(id = 2L, version = "1.0.11", versionCode = 12)
+
+        val conflicts = apkVersionService.detectVersionOrderConflicts(listOf(mistyped, newest))
+
+        assertEquals(1, conflicts.size)
+        assertEquals(newest, conflicts.first().newerName)
+        assertEquals(mistyped, conflicts.first().higherCode)
+    }
+
+    @Test
+    fun `detectVersionOrderConflicts should return empty when version codes follow version names`() {
+        val versions =
+            listOf(
+                versionOf(id = 3L, version = "1.0.11", versionCode = 12),
+                versionOf(id = 2L, version = "1.0.10", versionCode = 11),
+                versionOf(id = 1L, version = "1.0.9", versionCode = 10),
+            )
+
+        assertTrue(apkVersionService.detectVersionOrderConflicts(versions).isEmpty())
+    }
+
+    @Test
+    fun `detectVersionOrderConflicts should compare version name segments numerically`() {
+        val versions =
+            listOf(
+                versionOf(id = 1L, version = "1.0.9", versionCode = 9),
+                versionOf(id = 2L, version = "1.0.10", versionCode = 10),
+            )
+
+        assertTrue(apkVersionService.detectVersionOrderConflicts(versions).isEmpty())
+    }
+
+    @Test
+    fun `detectVersionOrderConflicts should report every conflicting pair`() {
+        val mistyped = versionOf(id = 1L, version = "1.0.10", versionCode = 100)
+        val versions =
+            listOf(
+                mistyped,
+                versionOf(id = 2L, version = "1.0.11", versionCode = 12),
+                versionOf(id = 3L, version = "1.1.0", versionCode = 13),
+            )
+
+        val conflicts = apkVersionService.detectVersionOrderConflicts(versions)
+
+        assertEquals(2, conflicts.size)
+        assertTrue(conflicts.all { it.higherCode == mistyped })
+        assertEquals(listOf("1.0.11", "1.1.0"), conflicts.map { it.newerName.version })
+    }
+
+    @Test
+    fun `detectVersionOrderConflicts should ignore versions that share a version name`() {
+        val versions =
+            listOf(
+                versionOf(id = 1L, version = "1.0.0", versionCode = 1),
+                versionOf(id = 2L, version = "1.0.0", versionCode = 2),
+            )
+
+        assertTrue(apkVersionService.detectVersionOrderConflicts(versions).isEmpty())
+    }
+
+    private fun versionOf(
+        id: Long,
+        version: String,
+        versionCode: Int,
+    ) = ApkVersionEntity(
+        id = id,
+        version = version,
+        versionCode = versionCode,
+        fileName = "kidspos-v$version.apk",
+        fileSize = 1000L,
+        filePath = "$testUploadDir/kidspos-v$version.apk",
+        isActive = true,
+        uploadedAt = LocalDateTime.now(),
+    )
+
     @Test
     fun `getVersionById should return version when exists`() {
         // Given
diff --git a/src/test/kotlin/info/nukoneko/kidspos/server/service/ReceiptServiceTest.kt b/src/test/kotlin/info/nukoneko/kidspos/server/service/ReceiptServiceTest.kt
index 04688df..aa41f98 100644
--- a/src/test/kotlin/info/nukoneko/kidspos/server/service/ReceiptServiceTest.kt
+++ b/src/test/kotlin/info/nukoneko/kidspos/server/service/ReceiptServiceTest.kt
@@ -10,6 +10,9 @@ import org.mockito.Mockito.verify
 import org.mockito.Mockito.`when`
 import org.springframework.boot.test.context.SpringBootTest
 import org.springframework.boot.test.mock.mockito.MockBean
+import java.net.ServerSocket
+import java.util.concurrent.ArrayBlockingQueue
+import java.util.concurrent.TimeUnit
 
 @SpringBootTest
 class ReceiptServiceTest {
@@ -157,4 +160,89 @@ class ReceiptServiceTest {
         assertTrue(result.contains("Change: 100リバー"))
         verify(storeService).findStore(storeId)
     }
+
+    @Test
+    fun `should not request printing when printer is not configured`() {
+        val storeId = 1
+        `when`(storeService.findStore(storeId)).thenReturn(StoreEntity(storeId, "Test Store", ""))
+
+        val service = receiptServiceWith(port = 9100)
+
+        assertFalse(service.printReceiptAsync(storeId, printableItems, 1000))
+    }
+
+    @Test
+    fun `should not request printing when store does not exist`() {
+        val storeId = 999
+        `when`(storeService.findStore(storeId)).thenReturn(null)
+
+        val service = receiptServiceWith(port = 9100)
+
+        assertFalse(service.printReceiptAsync(storeId, printableItems, 1000))
+    }
+
+    @Test
+    fun `should send receipt to printer in background`() {
+        val storeId = 1
+        val received = ArrayBlockingQueue<ByteArray>(1)
+
+        ServerSocket(0).use { server ->
+            Thread {
+                server.accept().use { socket ->
+                    received.put(socket.getInputStream().readBytes())
+                }
+            }.apply { isDaemon = true }.start()
+
+            `when`(storeService.findStore(storeId)).thenReturn(StoreEntity(storeId, "Test Store", "127.0.0.1"))
+            val service = receiptServiceWith(port = server.localPort)
+
+            assertTrue(service.printReceiptAsync(storeId, printableItems, 1000))
+
+            val bytes = received.poll(WAIT_LIMIT_MILLIS, TimeUnit.MILLISECONDS)
+            assertTrue(bytes != null && bytes.isNotEmpty(), "印字データが送信される")
+        }
+    }
+
+    @Test
+    fun `should return immediately when printer is unreachable`() {
+        val storeId = 1
+        `when`(storeService.findStore(storeId)).thenReturn(StoreEntity(storeId, "Test Store", UNROUTABLE_HOST))
+
+        val service = receiptServiceWith(port = 9100, connectTimeoutMillis = CONNECT_TIMEOUT_MILLIS)
+
+        val startedAt = System.nanoTime()
+        assertTrue(service.printReceiptAsync(storeId, printableItems, 1000))
+        val elapsedMillis = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startedAt)
+
+        assertTrue(elapsedMillis < CONNECT_TIMEOUT_MILLIS, "接続を待たずに返る: ${elapsedMillis}ms")
+    }
+
+    private fun receiptServiceWith(
+        port: Int,
+        connectTimeoutMillis: Int = CONNECT_TIMEOUT_MILLIS,
+    ): ReceiptService {
+        val properties =
+            AppProperties(
+                receipt =
+                    AppProperties.ReceiptProperties(
+                        printer =
+                            AppProperties.ReceiptProperties.PrinterProperties(
+                                port = port,
+                                connectTimeoutMillis = connectTimeoutMillis,
+                            ),
+                    ),
+            )
+        return ReceiptService(storeService, properties)
+    }
+
+    companion object {
+        private const val UNROUTABLE_HOST = "192.0.2.1"
+        private const val CONNECT_TIMEOUT_MILLIS = 3_000
+        private const val WAIT_LIMIT_MILLIS = 5_000L
+        private val printableItems =
+            listOf(
+                ItemBean(1, "0123456789", "あめ", 100),
+                ItemBean(2, "9876543210", "ガム", 200),
+            )
+    }
 }