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prerender-java

Jakarta Servlet Filter for 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.

Compatible with any Jakarta EE application server — Tomcat 10+, Jetty 11+, Spring Boot 3+, Quarkus, Micronaut.

Requires Java 17+.

Installation

Maven

<dependency>
    <groupId>io.prerender</groupId>
    <artifactId>prerender-java</artifactId>
    <version>1.0.0</version>
</dependency>

Gradle

implementation 'io.prerender:prerender-java:1.0.0'

Setup

Option 1: Environment variables (recommended)

export PRERENDER_TOKEN=your-token

Register the filter in web.xml:

<filter>
    <filter-name>PrerenderFilter</filter-name>
    <filter-class>io.prerender.PrerenderFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>PrerenderFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

Option 2: web.xml init-params

<filter>
    <filter-name>PrerenderFilter</filter-name>
    <filter-class>io.prerender.PrerenderFilter</filter-class>
    <init-param>
        <param-name>prerenderToken</param-name>
        <param-value>your-token</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>PrerenderFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

Spring Boot

@Bean
public FilterRegistrationBean<PrerenderFilter> prerenderFilter() {
    FilterRegistrationBean<PrerenderFilter> registration = new FilterRegistrationBean<>();
    registration.setFilter(new PrerenderFilter());
    registration.addUrlPatterns("/*");
    registration.setOrder(Ordered.HIGHEST_PRECEDENCE);
    return registration;
}

Set PRERENDER_TOKEN as an environment variable before starting the app.

Settings

Setting Init-param Env var Default
Token prerenderToken PRERENDER_TOKEN none
Service URL prerenderServiceUrl PRERENDER_SERVICE_URL https://service.prerender.io/

Init-params take precedence over environment variables.

Self-hosted Prerender

export PRERENDER_SERVICE_URL=http://your-prerender-server:3000

How it works

Requests are prerendered when all of the following are true:

  • The HTTP method is GET
  • The User-Agent matches a known bot/crawler (Googlebot, Bingbot, Twitterbot, GPTBot, ClaudeBot, etc.)
    — OR the URL contains _escaped_fragment_
    — OR the X-Bufferbot header is present
  • The URL does not end with a static asset extension (.js, .css, .png, etc.)

Everything else passes through to your normal servlet chain.

If the Prerender service is unreachable, the filter falls back gracefully and serves the normal response.

License

MIT