Apache Solr runs of Java 7 or greater, Java 8 is verified to be compatible and may bring some performance improvements. When using Oracle Java 7 or OpenJDK 7, be sure to not use the GA build 147 or update versions u40, u45 and u51! We recommend using u55 or later.
It is also recommended to always use the latest update version of your Java VM, because bugs may affect Solr. An overview of known JVM bugs can be found on http://wiki.apache.org/lucene-java/JavaBugs
With all Java versions it is strongly recommended to not use experimental -XX JVM options.
CPU, disk and memory requirements are based on the many choices made in implementing Solr (document size, number of documents, and number of hits retrieved to name a few). The benchmarks page has some information related to performance on particular platforms.