There are several libraries used JasperReports Java API for creating reports with Java:

• DynamicReports
• DynamicJasper

This libraries/frameworks can build reports "on fly" with or without using report's template (jrxml file)

To render fonts correctly in pdf font-extensions should always be used (in classpath)

• Subreports can be used for constructing complex reports. The reusing of existing reports is another goal of using subreports.

• The subreport will be shown as a part of master report in case using of <subreport> element.

• The value of subreportExpression parameter is differ for using at JasperReports Server or just by JasperReports framework (some API using or using in IDE).

  For JasperReports Server it looks like:

  <subreportExpression><![CDATA["repo:subreport.jrxml"]]></subreportExpression>
  

  For using by just JasperReports engine:

  <subreportExpression><![CDATA["/somePath/subreport.jasper"]]></subreportExpression>
  

  ^{The great explanation by @AndreasDietrich can be found at JasperServer: Unable to locate the subreport exception post}

• For some reasons the subreport can be used as a common report - without calling from the master report (with help of <subreport> element). The subreport is always a report.