SignApi

What it does:
With this api you can open a SignGui wtih predetermend text which can be read with the SignChangeEvent.

How it works:
The api scans the blocks at hight -63 in a square field(200x200) around the player. When a block at hight -63 is bedrock a sign is placed under it which is then edited and opened.

Custom event:
When the sign is closed by the player the api will call an custom event called CustomSignEditEvent.
-When this event is cancelled the sign will not be replaced with the previous material.
-When you set the previous material of the event (event.setPreviousMaterial(<Material>);) the sign will be replaced with this material.

Usage:
To use this api download it, add it to your buildpath.
I strongly recommend adding it also to your plugin folder for less code and safer approach.

Code examples:
Add Dependency to your plugin.yml:

depend: [SignApi]

Examplme code with api in plugin folder:

import org.bukkit.entity.Player;

import de.officeryoda.SignApi;

public class exampleClass {
 
    private void openSignGui(Player player, String[] lines) {
        if(lines.length == 0)
            SignApi.openNewSign(player, "text in line 1", "text in line 2", "text in line 3", "text in line 4");
        else
            SignApi.openNewSign(player, lines);
    }
}

Examplme code with api NOT in plugin folder:

import java.util.HashMap;
import java.util.Map;

import org.bukkit.Location;
import org.bukkit.Material;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.block.SignChangeEvent;

import de.officeryoda.SignApi;

public class exampleClass implements Listener {
    Map<Location, Material> signs;

    public exampleClass() {
        signs = new HashMap<>();
    }
 
    public void openSignGui(Player player, String[] lines) {
        if(lines.length == 0)
            signs.putAll(SignApi.openNewSign(player, "text in line 1", "text in line 2", "text in line 3", "text in line 4"));
        else
            signs.putAll(SignApi.openNewSign(player, lines));
    }
 
    @EventHandler
    public void onSignChange(SignChangeEvent event) {
        Location location = event.getBlock().getLocation();
        if(!signs.containsKey(location)) //checks if the sign is a sign placed by the plugin/api
            return;

        location.getWorld().getBlockAt(location).setType(signs.get(location)); //replaces the sign with the block which was there first
        signs.remove(location, signs.get(location)); //removes sign from list
    }
}

Read the sign with api in plugin folder:

import org.bukkit.ChatColor;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;

import de.officeryoda.CustomSignEditEvent;
import de.officeryoda.SignApi;

public class exampleClass implements Listener {

    public void openSignGui(Player player, String[] lines) {
        if(lines.length == 0)
            SignApi.openNewSign(player, "text of line 1", "text of line 2", "text of line 3", "text of line 3");
        else
            SignApi.openNewSign(player, lines);
    }

    @EventHandler
    public void onSignChange(CustomSignEditEvent event) {
        Player player = event.getPlayer(); //gets the player
        String[] lines = event.getLines(); //gets the lines

        for(int i = 0; i < lines.length; i++) {
            player.sendMessage(ChatColor.GRAY + "line "+ i + ": " + ChatColor.WHITE + lines[i]);
        }
        return;
    }
}

Read the sign with api NOT in plugin folder:

import java.util.HashMap;
import java.util.Map;

import org.bukkit.ChatColor;
import org.bukkit.Location;
import org.bukkit.Material;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.block.SignChangeEvent;

import de.officeryoda.SignApi;

public class exampleClass implements Listener {
    Map<Location, Material> signs;

    public exampleClass() {
        signs = new HashMap<>();
    }
 
    public void openSignGui(Player player, String[] lines) {
        if(lines.length == 0)
            signs.putAll(SignApi.openNewSign(player, "text of line 1", "text of line 2", "text of line 3", "text of line 3"));
        else
            signs.putAll(SignApi.openNewSign(player, lines));
    }
 
    @EventHandler
    public void onSignChange(SignChangeEvent event) {
        Player player = event.getPlayer(); //gets the player
        String[] lines = event.getLines(); //gets the lines
        Location location = event.getBlock().getLocation(); //gets the location

        if(signs.containsKey(location)) { //checks if the sign is a sign placed by the plugin/api
            for(int i = 0; i < lines.length; i++) {
                player.sendMessage(ChatColor.GRAY + "line "+ i + ": " + ChatColor.WHITE + lines[i]); //send the lines
            }
            return;
        }
        return;
    }
}