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Merge pull request #1 from prerender/add-prerender-java
Add Java Servlet filter
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‎.github/workflows/pull-request.yml‎

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name: Test
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on:
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pull_request:
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push:
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branches: [main]
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jobs:
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test:
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runs-on: ubuntu-latest
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strategy:
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fail-fast: false
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matrix:
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java-version: ['17', '21']
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steps:
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- uses: actions/checkout@v4
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- name: Setup Java
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uses: actions/setup-java@v4
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with:
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distribution: temurin
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java-version: ${{ matrix.java-version }}
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cache: maven
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- name: Setup Node (for contract mock server)
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uses: actions/setup-node@v4
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with:
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node-version: 20.x
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- name: Fetch contract mock server
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run: curl -fsSL -o mock-server.mjs https://raw.githubusercontent.com/prerender/integration-contract/main/mock-server.mjs
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- name: Run tests
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run: mvn --batch-mode --no-transfer-progress test

‎.gitignore‎

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target/
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*.iml
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.idea/
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.vscode/
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mock-server.mjs

‎LICENSE‎

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MIT License
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Copyright (c) 2026 Prerender
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Permission is hereby granted, free of charge, to any person obtaining a copy
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of this software and associated documentation files (the "Software"), to deal
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in the Software without restriction, including without limitation the rights
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to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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copies of the Software, and to permit persons to whom the Software is
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furnished to do so, subject to the following conditions:
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The above copyright notice and this permission notice shall be included in all
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copies or substantial portions of the Software.
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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SOFTWARE.

‎README.md‎

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# prerender-java
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Jakarta Servlet Filter for [Prerender.io](https://prerender.io). Intercepts requests from bots and crawlers and serves prerendered HTML, so your JavaScript-rendered app is fully indexable by search engines and social media scrapers.
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Compatible with any **Jakarta EE** application server — Tomcat 10+, Jetty 11+, Spring Boot 3+, Quarkus, Micronaut.
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Requires **Java 17+**.
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## Installation
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### Maven
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```xml
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<dependency>
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<groupId>io.prerender</groupId>
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<artifactId>prerender-java</artifactId>
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<version>1.0.0</version>
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</dependency>
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```
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### Gradle
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```groovy
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implementation 'io.prerender:prerender-java:1.0.0'
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```
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## Setup
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### Option 1: Environment variables (recommended)
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```bash
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export PRERENDER_TOKEN=your-token
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```
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Register the filter in `web.xml`:
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```xml
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<filter>
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<filter-name>PrerenderFilter</filter-name>
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<filter-class>io.prerender.PrerenderFilter</filter-class>
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</filter>
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<filter-mapping>
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<filter-name>PrerenderFilter</filter-name>
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<url-pattern>/*</url-pattern>
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</filter-mapping>
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```
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### Option 2: web.xml init-params
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```xml
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<filter>
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<filter-name>PrerenderFilter</filter-name>
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<filter-class>io.prerender.PrerenderFilter</filter-class>
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<init-param>
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<param-name>prerenderToken</param-name>
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<param-value>your-token</param-value>
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</init-param>
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</filter>
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<filter-mapping>
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<filter-name>PrerenderFilter</filter-name>
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<url-pattern>/*</url-pattern>
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</filter-mapping>
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```
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### Spring Boot
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```java
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@Bean
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public FilterRegistrationBean<PrerenderFilter> prerenderFilter() {
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FilterRegistrationBean<PrerenderFilter> registration = new FilterRegistrationBean<>();
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registration.setFilter(new PrerenderFilter());
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registration.addUrlPatterns("/*");
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registration.setOrder(Ordered.HIGHEST_PRECEDENCE);
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return registration;
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}
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```
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Set `PRERENDER_TOKEN` as an environment variable before starting the app.
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## Settings
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| Setting | Init-param | Env var | Default |
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|---------|------------|---------|---------|
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| Token | `prerenderToken` | `PRERENDER_TOKEN` | none |
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| Service URL | `prerenderServiceUrl` | `PRERENDER_SERVICE_URL` | `https://service.prerender.io/` |
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Init-params take precedence over environment variables.
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## Self-hosted Prerender
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```bash
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export PRERENDER_SERVICE_URL=http://your-prerender-server:3000
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```
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## How it works
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Requests are prerendered when **all** of the following are true:
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- The HTTP method is `GET`
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- The `User-Agent` matches a known bot/crawler (Googlebot, Bingbot, Twitterbot, GPTBot, ClaudeBot, etc.)
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— OR the URL contains `_escaped_fragment_`
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— OR the `X-Bufferbot` header is present
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- The URL does not end with a static asset extension (`.js`, `.css`, `.png`, etc.)
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Everything else passes through to your normal servlet chain.
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If the Prerender service is unreachable, the filter falls back gracefully and serves the normal response.
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## License
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MIT

‎pom.xml‎

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<?xml version="1.0" encoding="UTF-8"?>
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<project xmlns="http://maven.apache.org/POM/4.0.0"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<groupId>io.prerender</groupId>
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<artifactId>prerender-java</artifactId>
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<version>1.0.0</version>
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<packaging>jar</packaging>
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<name>prerender-java</name>
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<description>Jakarta Servlet Filter for prerendering JavaScript-rendered pages via Prerender.io</description>
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<url>https://github.com/prerender/integrations</url>
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<licenses>
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<license>
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<name>MIT License</name>
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<url>https://opensource.org/licenses/MIT</url>
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</license>
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</licenses>
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<scm>
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<connection>scm:git:[email protected]:prerender/integrations.git</connection>
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<developerConnection>scm:git:[email protected]:prerender/integrations.git</developerConnection>
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<url>https://github.com/prerender/integrations</url>
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</scm>
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<properties>
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<maven.compiler.source>17</maven.compiler.source>
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<maven.compiler.target>17</maven.compiler.target>
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<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
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</properties>
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<dependencies>
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<dependency>
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<groupId>jakarta.servlet</groupId>
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<artifactId>jakarta.servlet-api</artifactId>
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<version>6.0.0</version>
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<scope>provided</scope>
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</dependency>
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter</artifactId>
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<version>5.10.2</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.mockito</groupId>
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<artifactId>mockito-core</artifactId>
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<version>5.11.0</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.mockito</groupId>
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<artifactId>mockito-junit-jupiter</artifactId>
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<version>5.11.0</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.wiremock</groupId>
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<artifactId>wiremock</artifactId>
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<version>3.5.4</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>com.fasterxml.jackson.core</groupId>
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<artifactId>jackson-databind</artifactId>
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<version>2.17.0</version>
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<scope>test</scope>
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</dependency>
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</dependencies>
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<build>
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<plugins>
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-surefire-plugin</artifactId>
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<version>3.2.5</version>
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</plugin>
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</plugins>
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</build>
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</project>

‎src/main/java/io/prerender/PrerenderConfig.java‎

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package io.prerender;
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import java.util.List;
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class PrerenderConfig {
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static final List<String> CRAWLER_USER_AGENTS = List.of(
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"googlebot", "yahoo", "bingbot", "baiduspider",
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"facebookexternalhit", "twitterbot", "rogerbot", "linkedinbot",
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"embedly", "quora link preview", "showyoubot", "outbrain",
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"pinterest", "slackbot", "w3c_validator", "perplexity",
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"oai-searchbot", "chatgpt-user", "gptbot", "claudebot", "amazonbot"
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);
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static final List<String> EXTENSIONS_TO_IGNORE = List.of(
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".js", ".css", ".xml", ".less", ".png", ".jpg", ".jpeg", ".gif",
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".pdf", ".doc", ".txt", ".ico", ".rss", ".zip", ".mp3", ".rar",
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".exe", ".wmv", ".avi", ".ppt", ".mpg", ".mpeg", ".tif", ".wav",
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".mov", ".psd", ".ai", ".xls", ".mp4", ".m4a", ".swf", ".dat",
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".dmg", ".iso", ".flv", ".m4v", ".torrent", ".ttf", ".woff", ".svg"
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);
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private static final String DEFAULT_SERVICE_URL = "https://service.prerender.io/";
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private final String token;
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private final String serviceUrl;
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PrerenderConfig(String token, String serviceUrl) {
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this.token = token;
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this.serviceUrl = (serviceUrl != null && !serviceUrl.isBlank())
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? serviceUrl
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: DEFAULT_SERVICE_URL;
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}
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static PrerenderConfig fromInitParams(String initToken, String initServiceUrl) {
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return new PrerenderConfig(
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resolve(initToken, "PRERENDER_TOKEN"),
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resolve(initServiceUrl, "PRERENDER_SERVICE_URL")
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);
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}
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private static String resolve(String initParam, String envVar) {
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return (initParam != null && !initParam.isBlank()) ? initParam : System.getenv(envVar);
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}
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String getToken() { return token; }
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String getServiceUrl() { return serviceUrl; }
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static boolean isBot(String userAgent) {
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String ua = userAgent.toLowerCase();
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return CRAWLER_USER_AGENTS.stream().anyMatch(ua::contains);
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}
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static boolean isStaticAsset(String path) {
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String lower = path.toLowerCase();
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return EXTENSIONS_TO_IGNORE.stream().anyMatch(lower::endsWith);
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}
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}

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