feat: support for use through jvm -javaagent command line option

README.md

@@ -421,6 +421,23 @@ $ dcm -p 12345 getNegativePolicy
 $ dcm -p 12345 setNegativePolicy 0
 ```
 
+### Load Cache Entries from File
+
+```bash
+$ dcm -p 12345 load /foo/bar/my-cache.properties
+```
+
+### Manipulate cache with a command line option at JVM startup
+
+To manipulate dns entries at jvm startup, add the following jvm-options:
+
+```bash
+# Set individual entry
+java -javaagent:<path to jar>="set foo.com 1.1.1.1" --add-opens java.base/java.net=ALL-UNNAMED --add-opens java.base/sun.net=ALL-UNNAMED ...
+# Load entries from file
+java -javaagent:<path to jar>="load my-cache.properties" --add-opens java.base/java.net=ALL-UNNAMED --add-opens java.base/sun.net=ALL-UNNAMED ...
+```
+
 ## 📚 Related information
 
 * [Java Agent Specification](http://docs.oracle.com/javase/7/docs/api/java/lang/instrument/package-summary.html)

library/pom.xml

@@ -77,6 +77,7 @@
 				<configuration>
 					<archive>
 						<manifestEntries>
+							<Premain-Class>com.alibaba.dcm.agent.DcmAgent</Premain-Class>
 							<Agent-Class>com.alibaba.dcm.agent.DcmAgent</Agent-Class>
 							<Can-Redefine-Classes>false</Can-Redefine-Classes>
 							<Can-Retransform-Classes>false</Can-Retransform-Classes>

library/src/main/java/com/alibaba/dcm/DnsCacheManipulator.java

@@ -8,6 +8,7 @@
 
 import javax.annotation.Nullable;
 import javax.annotation.ParametersAreNonnullByDefault;
+import java.io.FileInputStream;
 import java.io.InputStream;
 import java.util.Arrays;
 import java.util.List;
@@ -144,6 +145,27 @@ public static void loadDnsCacheConfig(String propertiesFileName) {
         }
     }
 
+    /**
+     * Load dns config from the specified properties file in filesystem, then set dns cache.
+     *
+     * @param propertiesFileName specified properties file name in filesystem.
+     * @throws DnsCacheManipulatorException Operation fail
+     * @see DnsCacheManipulator#setDnsCache(java.util.Properties)
+     */
+    public static void loadDnsCacheConfigFromFileSystem(String propertiesFileName) {
+      try {
+        InputStream inputStream = new FileInputStream(propertiesFileName);
+        Properties properties = new Properties();
+        properties.load(inputStream);
+        inputStream.close();
+        setDnsCache(properties);
+      } catch (Exception e) {
+        final String message = String.format("Fail to loadDnsCacheConfig from %s, cause: %s",
+            propertiesFileName, e);
+        throw new DnsCacheManipulatorException(message, e);
+      }
+    }
+
     /**
      * Get a dns cache entry by {@code host}.
      *

library/src/main/java/com/alibaba/dcm/agent/DcmAgent.java

@@ -38,7 +38,14 @@ public class DcmAgent {
     static final String DCM_AGENT_SUCCESS_MARK_LINE = "!!DCM SUCCESS!!";
 
     /**
-     * Entrance method of DCM Java Agent.
+     * Entrance method of DCM Java Agent when used through a jvm command line option.
+     */
+    public static void premain(@Nonnull String agentArgument) throws Exception {
+      agentmain(agentArgument);
+    }
+
+    /**
+     * Entrance method of DCM Java Agent when connecting to a running jvm.
      */
     @SuppressFBWarnings("THROWS_METHOD_THROWS_CLAUSE_BASIC_EXCEPTION")
     public static void agentmain(@Nonnull String agentArgument) throws Exception {
@@ -264,6 +271,8 @@ private static synchronized void initAction2Method() throws NoSuchMethodExceptio
         map.put("ls", DnsCacheManipulator.class.getMethod("getWholeDnsCache"));
         map.put("clear", DnsCacheManipulator.class.getMethod("clearDnsCache"));
 
+        map.put("load", DnsCacheManipulator.class.getMethod("loadDnsCacheConfigFromFileSystem", String.class));
+
         map.put("setPolicy", DnsCacheManipulator.class.getMethod("setDnsCachePolicy", int.class));
         map.put("getPolicy", DnsCacheManipulator.class.getMethod("getDnsCachePolicy"));
         map.put("setNegativePolicy", DnsCacheManipulator.class.getMethod("setDnsNegativeCachePolicy", int.class));

library/src/test/java/com/alibaba/dcm/DnsCacheManipulatorTests.kt

@@ -3,15 +3,12 @@ package com.alibaba.dcm
 import com.alibaba.dcm.internal.InetAddressCacheUtilCommons.isNewInetAddressImpl
 import io.kotest.assertions.throwables.shouldThrow
 import io.kotest.core.spec.style.AnnotationSpec
-import io.kotest.matchers.collections.shouldBeEmpty
-import io.kotest.matchers.collections.shouldContainExactly
-import io.kotest.matchers.collections.shouldContainExactlyInAnyOrder
-import io.kotest.matchers.collections.shouldHaveSize
+import io.kotest.matchers.collections.*
 import io.kotest.matchers.longs.shouldBeBetween
 import io.kotest.matchers.nulls.shouldBeNull
 import io.kotest.matchers.shouldBe
 import io.kotest.matchers.types.shouldNotBeSameInstanceAs
-
+import kotlin.io.path.writeText
 
 private const val IP1 = "42.42.42.1"
 private const val IP2 = "42.42.42.2"
@@ -276,13 +273,13 @@ class DnsCacheManipulatorTests : AnnotationSpec() {
     }
 
     @Test
-    fun test_loadDnsCacheConfig_fromMyConfig() {
+    fun test_loadDnsCacheConfig_fromMyConfigClassPath() {
         DnsCacheManipulator.loadDnsCacheConfig("my-dns-cache.properties")
         "www.hello2.com".lookupIpByName() shouldBe IP2
     }
 
     @Test
-    fun test_multi_ips_in_config_file() {
+    fun test_multi_ips_in_config_file_classpath() {
         DnsCacheManipulator.loadDnsCacheConfig("dns-cache-multi-ips.properties")
 
         val host = "www.hello-multi-ips.com"
@@ -299,11 +296,32 @@ class DnsCacheManipulatorTests : AnnotationSpec() {
     }
 
     @Test
-    fun test_configNotFound() {
+    fun test_configNotFound_classpath() {
         val ex = shouldThrow<DnsCacheManipulatorException> {
             DnsCacheManipulator.loadDnsCacheConfig("not-existed.properties")
         }
 
         ex.message shouldBe "Fail to find not-existed.properties on classpath!"
     }
+
+    @Test
+    fun test_loadDnsCacheConfig_fromMyConfigFileSystem() {
+        val tempFile = kotlin.io.path.createTempFile("dns-cache-test", ".properties")
+        tempFile.writeText("www.hello2.com $IP2")
+        tempFile.toFile().deleteOnExit()
+
+        DnsCacheManipulator.loadDnsCacheConfigFromFileSystem(tempFile.toAbsolutePath().toString())
+        "www.hello2.com".lookupIpByName() shouldBe IP2
+    }
+
+    @Test
+    fun test_configNotFound_fileSystem() {
+        val ex = shouldThrow<DnsCacheManipulatorException> {
+            DnsCacheManipulator.loadDnsCacheConfigFromFileSystem("not-existed.properties")
+        }
+
+        ex.message shouldBeIn arrayOf(
+            "Fail to loadDnsCacheConfig from not-existed.properties, cause: java.io.FileNotFoundException: not-existed.properties (No such file or directory)",
+            "Fail to loadDnsCacheConfig from not-existed.properties, cause: java.io.FileNotFoundException: not-existed.properties (The system cannot find the file specified)")
+    }
 }