Fixed, I didn't understand the meaning of off-screen rendering at the beginning. Just render the depth value stored in the texture into the default framebuffer for display, and everything work just fine. I can't believe I spent 5 days on this.
gl.glBindFramebuffer(GL4.GL_FRAMEBUFFER, framebuff.get(0));
draw scene...(storing the depth value into the depth texture)
gl.glBindFramebuffer(GL4.GL_FRAMEBUFFER, 0);
draw scene...(comparing with the depth texture stored)