TestClass tc = new TestClass();
tc.getExtra().put("test", "value");
ParserContext ctx = new ParserContext();
ctx.withInput("this", TestClass.class);
ctx.setStrongTyping(true);
String expression = "extra[\"test\"]";
Serializable compiled = MVEL.compileSetExpression(expression, tc.getClass(), ctx);
MVEL.executeSetExpression(compiled, tc, "value3");
assertEquals("value3", tc.getExtra().get("test"));