Try to asynchronize some time-costly tasks

HMCLCore/src/main/kotlin/org/jackhuang/hmcl/game/DefaultGameRepository.kt

@@ -140,7 +140,6 @@ open class DefaultGameRepository(var baseDirectory: File): GameRepository {
         isLoaded = true
     }
 
-    @Synchronized
     final override fun refreshVersions() {
         EVENT_BUS.fireEvent(RefreshingVersionsEvent(this))
         refreshVersionsImpl()

HMCLCore/src/main/kotlin/org/jackhuang/hmcl/util/JavaVersion.kt

@@ -18,10 +18,12 @@
 package org.jackhuang.hmcl.util
 
 import com.google.gson.annotations.SerializedName
+import org.jackhuang.hmcl.game.Version
 import java.io.File
 import java.io.IOException
 import java.io.Serializable
 import java.util.*
+import java.util.concurrent.CountDownLatch
 import java.util.regex.Pattern
 
 /**
@@ -49,8 +51,6 @@ data class JavaVersion internal constructor(
     companion object {
         private val regex = Pattern.compile("java version \"(?<version>[1-9]*\\.[1-9]*\\.[0-9]*(.*?))\"")
 
-        val JAVAS: Map<String, JavaVersion>
-
         val UNKNOWN: Int = -1
         val JAVA_5: Int = 50
         val JAVA_6: Int = 60
@@ -132,7 +132,22 @@ data class JavaVersion internal constructor(
         }
         fun fromCurrentEnvironment() = currentJava
 
-        init {
+        private var javas: Map<String, JavaVersion>? = null
+        private val await = CountDownLatch(1)
+
+        /**
+         * This method will block until [initialize] succeeds.
+         */
+        fun getJREs(): Map<String, JavaVersion> {
+            if (javas != null) return javas!!
+            await.await()
+            return javas!!
+        }
+
+        @Synchronized
+        fun initialize() {
+            if (javas != null)
+                throw IllegalStateException("JavaVersions have already been initialized.")
             val temp = mutableMapOf<String, JavaVersion>()
             (when (OS.CURRENT_OS) {
                 OS.WINDOWS -> queryWindows()
@@ -141,7 +156,8 @@ data class JavaVersion internal constructor(
             }).forEach { javaVersion ->
                 temp.put(javaVersion.longVersion, javaVersion)
             }
-            JAVAS = temp
+            javas = temp
+            await.countDown()
         }
 
         private fun queryMacintosh() = LinkedList<JavaVersion>().apply {