使用Spring Boot新版后，ElasticSearch出现如下错误

Cannot register setting [http.netty.max_composite_buffer_components] twice

Caused by: java.lang.IllegalArgumentException: Cannot register setting [http.netty.max_composite_buffer_components] twice
    at org.elasticsearch.common.settings.SettingsModule.registerSetting(SettingsModule.java:172) ~[elasticsearch-5.6.16.jar:5.6.16]
    at org.elasticsearch.common.settings.SettingsModule.<init>(SettingsModule.java:74) ~[elasticsearch-5.6.16.jar:5.6.16]
    at org.elasticsearch.client.transport.TransportClient.buildTemplate(TransportClient.java:141) ~[elasticsearch-5.6.16.jar:5.6.16]
    at org.elasticsearch.client.transport.TransportClient.<init>(TransportClient.java:268) ~[elasticsearch-5.6.16.jar:5.6.16]
    at org.elasticsearch.transport.client.PreBuiltTransportClient.<init>(PreBuiltTransportClient.java:133) ~[transport-5.6.16.jar:5.6.16]
    at org.elasticsearch.transport.client.PreBuiltTransportClient.<init>(PreBuiltTransportClient.java:119) ~[transport-5.6.16.jar:5.6.16]
    at org.elasticsearch.transport.client.PreBuiltTransportClient.<init>(PreBuiltTransportClient.java:109) ~[transport-5.6.16.jar:5.6.16]

原因

因为transport使用的是5.5.3版本，对应的transport-netty3-client也是5.5.3，但是transport-netty4-client却被Spring Boot设置为6.4.3

解决办法

transport-netty4-client也设置为5.5.3

<dependency>
    <groupId>org.elasticsearch</groupId>
    <artifactId>elasticsearch</artifactId>
    <version>5.5.3</version>
</dependency>
<dependency>
    <groupId>org.elasticsearch.plugin</groupId>
    <artifactId>transport-netty4-client</artifactId>
    <version>5.5.3</version>
</dependency>
<dependency>
    <groupId>org.elasticsearch.client</groupId>
    <artifactId>transport</artifactId>
    <version>5.5.3</version>
    <exclusions>
        <exclusion>
            <groupId>*</groupId>
            <artifactId>transport-netty4-client</artifactId>
        </exclusion>
    </exclusions>
</dependency>

版本不兼容原因分析

在两个版本中，Netty3Plugin和Netty4Plugin都会去设置属性http.netty.max_composite_buffer_components，但是区别在于5.5.3中，这个属性是共享的Property.Shared，但是在6.4.3中该属性不是共享（其实共享的本意我还没有弄得很清楚）

• 5.5.3
  {Property.NodeScope, Property.Shared}

SETTING_HTTP_NETTY_MAX_COMPOSITE_BUFFER_COMPONENTS = Setting.intSetting("http.netty.max_composite_buffer_components", -1, new Property[]{Property.NodeScope, Property.Shared});

• 6.4.3
  Property.NodeScope

public static Setting<Integer> SETTING_HTTP_NETTY_MAX_COMPOSITE_BUFFER_COMPONENTS =
    new Setting<>(SETTING_KEY_HTTP_NETTY_MAX_COMPOSITE_BUFFER_COMPONENTS, (s) -> {
        ByteSizeValue maxContentLength = SETTING_HTTP_MAX_CONTENT_LENGTH.get(s);
        long maxBufferComponentsEstimate = Math.round((double) (maxContentLength.getBytes() / MTU.getBytes()));
        long maxBufferComponents = Math.max(2, Math.min(maxBufferComponentsEstimate, Integer.MAX_VALUE));
        return String.valueOf(maxBufferComponents);
    }, s -> Setting.parseInt(s, 2, Integer.MAX_VALUE, SETTING_KEY_HTTP_NETTY_MAX_COMPOSITE_BUFFER_COMPONENTS), Property.NodeScope);