1. 在啟動腳本中使用-bootstrap或-Xbootclasspath選項

這兩個選項的使用方式如下：

-bootstrap選項：

java -bootstrap /path/to/your.jar -cp /path/to/your/app.jar YourMainClass

-Xbootclasspath選項：

java -Xbootclasspath/a:/path/to/your.jar -cp /path/to/your/app.jar YourMainClass

請注意，-bootstrap選項在某些Java版本中可能不受支持，而-Xbootclasspath選項通常在大多數(shù)Java虛擬機中可用。

2. 通過manifest file(jar包META-INF/MANIFEST.MF目錄下)中的Boot-Class-Path屬性實現(xiàn)

Maven項目中，您可以通過使用maven-jar-plugin插件來配置JAR文件的Manifest屬性。下面是如何配置Manifest屬性的一般步驟：

• 打開項目的pom.xml文件。
• 在build元素下，添加plugins元素，如果尚不存在的話。然后在plugins元素內(nèi)部配置maven-jar-plugin插件。示例如下：

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-jar-plugin</artifactId>
            <version>2.3.2</version>
            <configuration>
                <archive>
                        <manifestEntries>
                            <Premain-Class>com.br.prometheus.SPSExporter</Premain-Class>
                            <Boot-Class-Path>${project.build.finalName}.jar</Boot-Class-Path>
                            <Can-Redefine-Classes>false</Can-Redefine-Classes>
                            <Can-Retransform-Classes>true</Can-Retransform-Classes>
                            <Can-Set-Native-Method-Prefix>false</Can-Set-Native-Method-Prefix>
                        </manifestEntries>
                </archive>
            </configuration>
        </plugin>
        <!-- 其他插件配置 -->
    </plugins>
</build>

在上面的示例中，我們配置了maven-jar-plugin插件，并在<manifestEntries>元素下添加了一些屬性。其中：

Manifest Attributes

The following manifest attributes are defined for an agent JAR file:

Premain-Class

When an agent is specified at JVM launch time this attribute specifies the agent class. That is, the class containing the premain method. When an agent is specified at JVM launch time this attribute is required. If the attribute is not present the JVM will abort. Note: this is a class name, not a file name or path.

Agent-Class

If an implementation supports a mechanism to start agents sometime after the VM has started then this attribute specifies the agent class. That is, the class containing the agentmain method. This attribute is required, if it is not present the agent will not be started. Note: this is a class name, not a file name or path.

Boot-Class-Path

A list of paths to be searched by the bootstrap class loader. Paths represent directories or libraries (commonly referred to as JAR or zip libraries on many platforms). These paths are searched by the bootstrap class loader after the platform specific mechanisms of locating a class have failed. Paths are searched in the order listed. Paths in the list are separated by one or more spaces. A path takes the syntax of the path component of a hierarchical URI. The path is absolute if it begins with a slash character ('/'), otherwise it is relative. A relative path is resolved against the absolute path of the agent JAR file. Malformed and non-existent paths are ignored. When an agent is started sometime after the VM has started then paths that do not represent a JAR file are ignored. This attribute is optional.

Can-Redefine-Classes

Boolean (true or false, case irrelevant). Is the ability to redefine classes needed by this agent. Values other than true are considered false. This attribute is optional, the default is false.

Can-Retransform-Classes

Boolean (true or false, case irrelevant). Is the ability to retransform classes needed by this agent. Values other than true are considered false. This attribute is optional, the default is false.

Can-Set-Native-Method-Prefix

Boolean (true or false, case irrelevant). Is the ability to set native method prefix needed by this agent. Values other than true are considered false. This attribute is optional, the default is false.

An agent JAR file may have both the Premain-Class and Agent-Class attributes present in the manifest. When the agent is started on the command-line using the -javaagent option then the Premain-Class attribute specifies the name of the agent class and the Agent-Class attribute is ignored. Similarly, if the agent is started sometime after the VM has started, then the Agent-Class attribute specifies the name of the agent class (the value of Premain-Class attribute is ignored).

保存pom.xml文件。

使用Maven命令構建項目。您可以運行以下命令來生成包含指定Manifest屬性的JAR文件：

mvn clean package

這將生成一個JAR文件，其中包含了配置的Manifest屬性。

Manifest-Version: 1.0
Archiver-Version: Plexus Archiver
Created-By: Apache Maven
Built-By: mingming.chen
Build-Jdk: 1.8.0_211
Boot-Class-Path: sps_exporter.jar
Can-Redefine-Classes: false
Can-Retransform-Classes: true
Can-Set-Native-Method-Prefix: false
Premain-Class: com.br.prometheus.SPSExporter

通過這種方式，您可以方便地配置JAR文件的Manifest屬性，包括類路徑、主類和其他自定義屬性。請根據(jù)您的項目需求進行相應的配置。

通過以上方式java agent可以字節(jié)碼修改jdk中的類

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