1. received plaintext http traffic on an https channel, closing connection Netty4HttpChannel

[2022-02-16T21:08:50,085][WARN ][o.e.x.s.t.n.SecurityNetty4HttpServerTransport] [DESKTOP-VCT39JM] received plaintext http traffic on an https channel, closing connection Netty4HttpChannel{localAddress=/[0:0:0:0:0:0:0:1]:9200, remoteAddress=/[0:0:0:0:0:0:0:1]:1172}

解决

是因为开启了 ssl 认证。
在 ES/config/elasticsearch.yml 文件中把 xpack.security.http.ssl:enabled 设置成 false 即可

# Enable encryption for HTTP API client connections, such as Kibana, Logstash, and Agents
xpack.security.http.ssl:
  enabled: false
  keystore.path: certs/http.p12

2. elasticsearch 账号密码

windows 下直接启动 ElasticSearch ，见到 started 为成功启动，访问 htttp://localhost:9200 需要输入密码，是因为开启了密码验证模式。
找了一轮没看到有账号密码，干脆就设置免密登录就好。

解决

找到 elasticsearch.yml 文件， 把 xpack.security.enabled 属性设置为 false 即可。

# Enable security features
xpack.security.enabled: false

3. 设置内存大小

ES 的内存是自己调节的。在 config/jvm.options 文件中直接设置就好(追加):

-Xms512m
-Xmx2048m

4. windows Could not rename log file ‘logs/gc.log’ to ‘logs/gc.log.14’ (Permission denied).

ES 在 windows 中只允许打开一个应用程序，当你再想去创建一个 ES 应用程序的时候，就会显示 Permission denied，即使是使用 cmd 管理员运行 elasticsearch.bat 文件也是一样的错误。

• 解决： 注意查看是否已经在别的地方已经打开 ES 服务，实在不行则进行电脑重启

5. org/elasticsearch/action/ActionRequest has been compiled by a more recent version of the Java Runtime (class file version 61.0), this version of the Java Runtime only recognizes class file versions up to 52.0

public static void main(String[] args) throws IOException {
    // 创建 ES 客户端
    RestHighLevelClient client = new RestHighLevelClient(
        RestClient.builder(new HttpHost(Constants.HOST, Constants.PORT, Constants.HTTP))
    );

    client.close();
}

通过上述代码，使用 RestHighLevelCilent 访问 ES 客户端的时候，出现以下错误：Exception in thread "main" java.lang.UnsupportedClassVersionError:org/elasticsearch/action/ActionRequest has been compiled by a more recent version of the Java Runtime (class file version 61.0), this version of the Java Runtime only recognizes class file versions up to 52.0

看到注释，我们知道是版本不兼容的问题，查找资料看到这样一个表：

Major version numbers map to Java versions:

45 = Java 1.1
46 = Java 1.2
47 = Java 1.3
48 = Java 1.4
49 = Java 5
50 = Java 6
51 = Java 7
52 = Java 8
53 = Java 9
54 = Java 10
55 = Java 11
56 = Java 12
57 = Java 13

讲道理在这个版本任你发，我用Java8 的年代，RestHighLevelClient 肯定是兼容Java 8 的，那么就只有 ES 版本太高了，把 pom.xml 的 ES 依赖版本降到跟 elasticsearch-rest-high-level-client 一样就可以了

<properties>
    <elasticsearch.version>7.17.0</elasticsearch.version>   	 																	<elasticsearch.client.version>7.17.0</elasticsearch.client.version>
</properties>

<dependies>
    <dependency>
        <groupId>org.elasticsearch</groupId>
        <artifactId>elasticsearch</artifactId>
        <version>${elasticsearch.version}</version>
    </dependency>
    <dependency>
        <groupId>org.elasticsearch.client</groupId>
        <artifactId>elasticsearch-rest-high-level-client</artifactId>
        <version>${elasticsearch.client.version}</version>
    </dependency>
</dependies>

• 解决：把 pom.xml 的 elasticsearch 和 elasticsearch-rest-high-level-client 版本一致即可。