Check java version according to manifest

picomc/instance.py

@@ -123,7 +123,7 @@ def launch(self, account, version=None, verify_hashes=False):
         os.makedirs(gamedir, exist_ok=True)
 
         java = self.get_java()
-        java_info = assert_java(java)
+        java_info = assert_java(java, vobj.java_version)
 
         libraries = vobj.get_libraries(java_info)
         vobj.prepare_launch(gamedir, java_info, verify_hashes)
@@ -146,7 +146,7 @@ def launch(self, account, version=None, verify_hashes=False):
 
     def extract_natives(self):
         vobj = self.launcher.version_manager.get_version(self.config["version"])
-        java_info = assert_java(self.get_java())
+        java_info = assert_java(self.get_java(), vobj.java_version)
         vobj.download_libraries(java_info, verify_hashes=True)
         libs = vobj.get_libraries(java_info)
         ne = NativesExtractor(

picomc/java/javainfo.py

@@ -46,10 +46,31 @@ def get_java_info(java):
     return res
 
 
-def assert_java(java):
+def check_version_against(version: str, wanted):
+    component = wanted["component"]
+    wanted_major = str(wanted["majorVersion"])
+
+    major, minor, *_ = version.split(".")
+
+    if component == "jre-legacy":
+        return major == "1" and minor == wanted_major
+    else:
+        return major == wanted_major
+
+
+def wanted_to_str(wanted):
+    component = wanted["component"]
+    major = str(wanted["majorVersion"])
+
+    if component == "jre-legacy":
+        return f"1.{major}.0"
+    else:
+        return str(major)
+
+
+def assert_java(java, wanted):
     try:
         jinfo = get_java_info(java)
-        badjv = not jinfo["java.version"].startswith("1.8.0")
         bitness = jinfo.get("sun.arch.data.model", None)
         if bitness and bitness != "64":
             logger.warning(
@@ -62,12 +83,24 @@ def assert_java(java):
             )
         )
 
-        if badjv:
+        if not check_version_against(jinfo["java.version"], wanted):
             logger.warning(
-                "Minecraft uses java 1.8.0 by default."
-                " You may experience issues, especially with older versions of Minecraft."
+                "The version of Minecraft you are launching "
+                "uses java {} by default.".format(wanted_to_str(wanted))
             )
 
+            logger.warning(
+                "You may experience issues, especially with older versions of Minecraft."
+            )
+
+            major = int(jinfo["java.version"].split(".")[0])
+            if major < wanted["majorVersion"]:
+                logger.error(
+                    "Note that at least java {} is required to launch at all.".format(
+                        wanted_to_str(wanted)
+                    )
+                )
+
         return jinfo
 
     except FileNotFoundError:

picomc/version.py

@@ -70,6 +70,11 @@ def argumentadd(d1, d2):
     ),
 }
 
+LEGACY_JAVA_VERSION = {
+    "component": "jre-legacy",
+    "majorVersion": 8,
+}
+
 
 class VersionSpec:
     def __init__(self, vobj, version_manager):
@@ -109,6 +114,12 @@ def initialize_fields(self):
             self.arguments = self.attr_reduce("arguments", argumentadd)
         except AttributeError:
             pass
+
+        # TODO maybe we could detect extremely old versions of Minecraft and
+        # put an appropriate version of java here in that case
+        self.javaVersion = self.attr_override(
+            "javaVersion", default=LEGACY_JAVA_VERSION
+        )
         self.mainClass = self.attr_override("mainClass")
         self.assetIndex = self.attr_override("assetIndex", default=None)
         self.assets = self.attr_override("assets", default="legacy")
@@ -141,6 +152,8 @@ def __init__(self, version_name, launcher, version_manifest):
         self.jarname = self.vspec.jar
         self.jarfile = self.versions_root / self.jarname / "{}.jar".format(self.jarname)
 
+        self.java_version = self.vspec.javaVersion
+
     def get_raw_vspec(self):
         vspec_path = (
             self.versions_root / self.version_name / "{}.json".format(self.version_name)