[BUG][Client][Java 21][WebClient] Generated code contains warnings on java 21

[EdmonDantes]

Bug Report Checklist

• [+] Have you provided a full/minimal spec to reproduce the issue?
• [+] Have you validated the input using an OpenAPI validator (example)?
• [+] Have you tested with the latest master to confirm the issue still exists?
• [+] Have you searched for related issues/PRs?
• [+] What's the actual output vs expected output?
• [+] [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Generator generates code with this-escape warnings in ApiClient.
If you add -Xlint:all, -Werror to java compiler options, you will get error on any open-api specification.

Java compiler logs

...\project\build\java\src\main\java\org\openapitools\example\invoker\ApiClient.java:103: warning: [this-escape] possible 'this' escape before subclass is fully initialized
        this.init();
                 ^
...\project\build\java\src\main\java\org\openapitools\example\invoker\ApiClient.java:107: warning: [this-escape] possible 'this' escape before subclass is fully initialized
        this(Optional.ofNullable(webClient).orElseGet(() -> buildWebClient()), createDefaultDateFormat());
            ^
...\project\build\java\src\main\java\org\openapitools\example\invoker\ApiClient.java:122: warning: [this-escape] previous possible 'this' escape happens here via invocation
        this.init();
                 ^
...\project\build\java\src\main\java\org\openapitools\example\invoker\ApiClient.java:111: warning: [this-escape] possible 'this' escape before subclass is fully initialized
        this(buildWebClient(mapper.copy()), format);
            ^
...\project\build\java\src\main\java\org\openapitools\example\invoker\ApiClient.java:122: warning: [this-escape] previous possible 'this' escape happens here via invocation
        this.init();
                 ^
...\project\build\java\src\main\java\org\openapitools\example\invoker\ApiClient.java:115: warning: [this-escape] possible 'this' escape before subclass is fully initialized
        this(Optional.ofNullable(webClient).orElseGet(() -> buildWebClient(mapper.copy())), format);
            ^
...\project\build\java\src\main\java\org\openapitools\example\invoker\ApiClient.java:122: warning: [this-escape] previous possible 'this' escape happens here via invocation
        this.init();
                 ^

openapi-generator version

v7.2.0

OpenAPI declaration file content or URL

https://github.com/OpenAPITools/openapi-generator/blob/1105759a6f20edac389fa547359c7d5e6c4f9ac8/modules/openapi-generator-gradle-plugin/samples/local-spec/petstore-v3.0.yaml

Information to reproduce

jdk: Oracle OpenJDK 21.0.1
gradle: 8.5
open-api generator plugin: 7.2.0

build.gradle.kts

plugins {
    id("java")
    id("org.openapi.generator") version "7.2.0"
}

group = "test.group"
version = "1.0"

repositories {
    mavenCentral()
}

dependencies {
    implementation("org.openapitools:jackson-databind-nullable:0.2.6")
    implementation("com.fasterxml.jackson.core:jackson-annotations:2.16.1")
    implementation("com.fasterxml.jackson.core:jackson-core:2.16.1")
    implementation("com.fasterxml.jackson.core:jackson-databind:2.16.1")
    implementation("org.springframework.boot:spring-boot-starter-webflux:3.2.1")
    testImplementation(platform("org.junit:junit-bom:5.9.1"))
    testImplementation("org.junit.jupiter:junit-jupiter")
}

sourceSets {
    named("main") {
        java {
            srcDir("$buildDir/java/src/main/java")
        }
    }
}

tasks.test {
    useJUnitPlatform()
}

openApiGenerate {
    generatorName = "java"
    library = "webclient"
    inputSpec = "$projectDir/src/main/openapi/petstore-v3.0.yaml"
    outputDir = "$buildDir/java"
    apiPackage = "org.openapitools.example.api"
    invokerPackage = "org.openapitools.example.invoker"
    modelPackage = "org.openapitools.example.model"
    configOptions.putAll(mapOf(
        "dateLibrary" to "java8",
        "serializationLibrary" to "jackson",
        "useJakartaEe" to "true",
        "hideGenerationTimestamp" to "true"
    ))
    globalProperties.putAll(mapOf(
        "apiTests" to "false",
        "modelDocs" to "false"
    ))
    skipValidateSpec = true
    logToStderr = true
    generateAliasAsModel = false
    enablePostProcessFile = false
}

tasks.withType<JavaCompile>() {
    options.compilerArgs.addAll(listOf("-Xlint:all", "-Werror"))
}

Suggest a fix

Change init method in class ApiClient