class Demo {
            private String test;
            private String _test_test;

            public String get_test_test () {
                return _test_test;
            }

            public void set_test_test (String _test_test) {
                this._test_test = _test_test;
            }

            public String getTest () {
                return test;
            }

            public void setTest (String test) {
                this.test = test;
            }
        }
        Demo demo = new Demo();
        demo.setTest("TEST");
        demo.set_test_test("TEST_TEST");
        String text = JSON.toJSONString(demo);
        Demo obj = JSON.parseObject(text, Demo.class);
        Assert.assertEquals("TEST",obj.getTest());
        Assert.assertEquals("TEST_TEST",obj.get_test_test());

我按照你的例子试了下，没有问题，fastjson 1.2.36