Support being called as Java-Agent

[zbx1425]

Support being called as Java-Agent would enable Packwiz-Installer to be used on:

• Launchers that has incorrect handling of $INST_JAVA, meaning they can't start packwiz-installer from pre-launch commands reliably (HMCL / PCL2)
• Launchers that don't support pre-launch commands (Mojang Launcher, IIRC?)

---

Currently there's a community workaround for the aforementioned issues ("packwiz-installer-bootstrap-wrapper") by teacon.org:

package org.teacon.codenamed;

import java.io.File;
import java.lang.instrument.Instrumentation;

public class JavaAgentEntryPoint {

  public static void premain(String args, Instrumentation inst) {
    String javaExecutable = System.getProperty("java.home") + System.getProperty("java.home") + "bin" + File.separator;
    if (System.getProperty("os.name").startsWith("Win")) {
      javaExecutable = javaExecutable + "javaw.exe";
    } else {
      javaExecutable = javaExecutable + "java";
    } 
    try {
      Process p = (new ProcessBuilder(new String[] { javaExecutable, "-jar", "packwiz-installer-bootstrap.jar", "--bootstrap-main-jar", "packwiz-installer.jar", "--bootstrap-no-update", args })).inheritIO().start();
      int exitCode;
      if ((exitCode = p.waitFor()) != 0)
        throw new RuntimeException("Process exit status is " + exitCode + ", meaning an error has occurred."); 
    } catch (Exception e) {
      throw new RuntimeException("Game failed to launch, see console output for details.", e);
    } 
  }
}

[3944Realms]

There is indeed an error here. In the Java code, System.getProperty("java.home") is called twice, which will lead to a string concatenation error. The correct approach should be to call System.getProperty("java.home") only once, and then concatenate it with "bin" and File.separator. Here is the corrected code:

String javaExecutable = System.getProperty("java.home") + "bin" + File.separator;

[3944Realms]

I believe the correct code is:

package org.teacon.codenamed;

import java.lang.instrument.Instrumentation;
import java.nio.file.Paths;

public class JavaAgentEntryPoint {

    public static void premain(String args, Instrumentation inst) {
        String javaExecutable = Paths.get(
                System.getProperty("java.home"),
                "bin",
                System.getProperty("os.name").startsWith("Win") ? "javaw.exe" : "java"
        ).toString();

        System.out.println("[JavaAgentEntryPoint] Using Java executable: " + javaExecutable);

        try {
            Process p = new ProcessBuilder(
                    javaExecutable,
                    "-jar", "packwiz-installer-bootstrap.jar",
                    "--bootstrap-main-jar", "packwiz-installer.jar",
                    "--bootstrap-no-update",
                    args
            ).inheritIO().start();

            int exitCode = p.waitFor();
            if (exitCode != 0) {
                throw new RuntimeException("Process exit status is " + exitCode + ", meaning an error has occurred.");
            }
        } catch (Exception e) {
            throw new RuntimeException("Game failed to launch, see console output for details.", e);
        }
    }
}