Merge pull request #1565 from JaredTan95/legacy

Support to override meta server url through environment variables.

apollo-core/src/main/java/com/ctrip/framework/apollo/core/internals/LegacyMetaServerProvider.java

 package com.ctrip.framework.apollo.core.internals;
 
-import java.util.HashMap;
-import java.util.Map;
-import java.util.Properties;
-
 import com.ctrip.framework.apollo.core.enums.Env;
 import com.ctrip.framework.apollo.core.spi.MetaServerProvider;
 import com.ctrip.framework.apollo.core.utils.ResourceUtils;
+import com.google.common.base.Strings;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Properties;
 
 /**
  * For legacy meta server configuration use, i.e. apollo-env.properties
  */
 public class LegacyMetaServerProvider implements MetaServerProvider {
+
   // make it as lowest as possible, yet not the lowest
   public static final int ORDER = MetaServerProvider.LOWEST_PRECEDENCE - 1;
   private static final Map<Env, String> domains = new HashMap<>();
@@ -23,19 +24,27 @@ public class LegacyMetaServerProvider implements MetaServerProvider {
   private void initialize() {
     Properties prop = new Properties();
     prop = ResourceUtils.readConfigFile("apollo-env.properties", prop);
-    Properties env = System.getProperties();
-    domains.put(Env.LOCAL,
-        env.getProperty("local_meta", prop.getProperty("local.meta")));
-    domains.put(Env.DEV,
-        env.getProperty("dev_meta", prop.getProperty("dev.meta")));
-    domains.put(Env.FAT,
-        env.getProperty("fat_meta", prop.getProperty("fat.meta")));
-    domains.put(Env.UAT,
-        env.getProperty("uat_meta", prop.getProperty("uat.meta")));
-    domains.put(Env.LPT,
-        env.getProperty("lpt_meta", prop.getProperty("lpt.meta")));
-    domains.put(Env.PRO,
-        env.getProperty("pro_meta", prop.getProperty("pro.meta")));
+
+    domains.put(Env.LOCAL, getMetaServerAddress(prop, "local_meta", "local.meta"));
+    domains.put(Env.DEV, getMetaServerAddress(prop, "dev_meta", "dev.meta"));
+    domains.put(Env.FAT, getMetaServerAddress(prop, "fat_meta", "fat.meta"));
+    domains.put(Env.UAT, getMetaServerAddress(prop, "uat_meta", "uat.meta"));
+    domains.put(Env.LPT, getMetaServerAddress(prop, "lpt_meta", "lpt.meta"));
+    domains.put(Env.PRO, getMetaServerAddress(prop, "pro_meta", "pro.meta"));
+  }
+
+  private String getMetaServerAddress(Properties prop, String sourceName, String propName) {
+    // 1. Get from System Property.
+    String metaAddress = System.getProperty(sourceName);
+    if (Strings.isNullOrEmpty(metaAddress)) {
+      // 2. Get from OS environment variable, which could not contain dot and is normally in UPPER case,like DEV_META.
+      metaAddress = System.getenv(sourceName.toUpperCase());
+    }
+    if (Strings.isNullOrEmpty(metaAddress)) {
+      // 3. Get from properties file.
+      metaAddress = prop.getProperty(propName);
+    }
+    return metaAddress;
   }
 
   @Override