问题描述
在部署一个验证码服务的容器服务时遇到了一个空指针错误，错误代码为：
java.lang.NullPointerException
at sun.awt.FontConfiguration.getVersion(FontConfiguration.java:1264)
at sun.awt.FontConfiguration.readFontConfigFile(FontConfiguration.java:219)
at sun.awt.FontConfiguration.init(FontConfiguration.java:107)
at sun.awt.X11FontManager.createFontConfiguration(X11FontManager.java:774)
at sun.font.SunFontManager$2.run(SunFontManager.java:431)
at java.security.AccessController.doPrivileged(Native Method)
at sun.font.SunFontManager.<init>(SunFontManager.java:376)
at sun.awt.FcFontManager.<init>(FcFontManager.java:35)
at sun.awt.X11FontManager.<init>(X11FontManager.java:57)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at java.lang.Class.newInstance(Class.java:442)
at sun.font.FontManagerFactory$1.run(FontManagerFactory.java:83)
at java.security.AccessController.doPrivileged(Native Method)
at sun.font.FontManagerFactory.getInstance(FontManagerFactory.java:74)
at java.awt.Font.getFont2D(Font.java:491)
at java.awt.Font.access$000(Font.java:224)
at java.awt.Font$FontAccessImpl.getFont2D(Font.java:228)
at sun.font.FontUtilities.getFont2D(FontUtilities.java:180)
at sun.font.StandardGlyphVector.initFontData(StandardGlyphVector.java:1126)
at sun.font.StandardGlyphVector.init(StandardGlyphVector.java:1115)
at sun.font.StandardGlyphVector.<init>(StandardGlyphVector.java:167)
at java.awt.Font.createGlyphVector(Font.java:2545)
at nl.captcha.text.renderer.DefaultWordRenderer.render(Unknown Source)
at nl.captcha.Captcha$Builder.addText(Unknown Source)
at com.liferay.portal.captcha.simplecaptcha.SimpleCaptchaImpl.getSimpleCaptcha(SimpleCaptchaImpl.java:243)
at com.liferay.portal.captcha.simplecaptcha.SimpleCaptchaImpl.serveImage(SimpleCaptchaImpl.java:159)
at com.liferay.portal.captcha.CaptchaImpl.serveImage(CaptchaImpl.java:100)
at com.liferay.portal.kernel.captcha.CaptchaUtil.serveImage(CaptchaUtil.java:78)
at com.liferay.portal.captcha.CaptchaPortletAction.serveResource(CaptchaPortletAction.java:42)
在仔细搜索了一番后，发现github上的一个描述比较相似：
https://github.com/docker-library/openjdk/issues/73
解决方法
经过了解，知道是因为alpine中缺少FontConfiguration，那么就考虑安装ttf-dejavu这个软件。
在http://blog.csdn.net/freewebsys/article/details/53816615找到了解决办法。
在服务器适当位置位置创建一个Dockerfile，内容为：
FROM java:8-jre-alpine
# Install cURL
RUN echo -e "https://mirror.tuna.tsinghua.edu.cn/alpine/v3.4/main\n\
https://mirror.tuna.tsinghua.edu.cn/alpine/v3.4/community" > /etc/apk/repositories
RUN apk --update add curl bash ttf-dejavu && \
rm -rf /var/cache/apk/*
在同一位置执行：
docker build -t docker.io/java-font:8-jre-alpine .
执行完毕后会有一个新的java的镜像：
REPOSITORY IMAGE ID CREATED SIZE
docker.io/java-font 8-jre-alpine dc7703ec6f07 31 hours ago 131.5 MB
再次构建项目的时候使用新的jdk镜像即可解决问题。
