java.time

The java.time framework built into Java 8 and later has a Duration.parse method for parsing an ISO 8601 formatted duration:

java.time.Duration d = java.time.Duration.parse("PT1H2M34S");
System.out.println("Duration in seconds: " + d.get(java.time.temporal.ChronoUnit.SECONDS));

Prints Duration in seconds: 3754