java.lang.NoSuchMethodError 介绍

1、错误内容：java.lang.NoSuchMethodError: com.Boot: method <init>()V not found

此提示内容指，com.Boot没有参数为空的构造函数。提示中指明了异常所在的类和对应的函数。

2、 java文档：说的很清楚了，是类的变化不兼容导致的异常。通俗的说法是，调用方使用的类定义和加载的类定义不一样，加载的类实际上没有将要调用的方法。

/**
 * Thrown if an application tries to call a specified method of a
 * class (either static or instance), and that class no longer has a
 * definition of that method.
 * <p>
 * Normally, this error is caught by the compiler; this error can
 * only occur at run time if the definition of a class has
 * incompatibly changed.
 *
 * @author  unascribed
 * @since   JDK1.0
 */
public
class NoSuchMethodError extends IncompatibleClassChangeError {

3、如何制造一个NoSuchMethodError：

这里提供两个java代码。放在com目录下。依次执行以下命令：

javac com/Boot.java

javac com/Cloud.java

vim com/Boot.java  把Boot的无参构造函数注释掉，把有参构造函数的注释打开

javac com/Boot.java  重新编译Boot的类文件，最终结果没有无参构造函数

java com/Cloud  此处会出现NoSuchMethodError异常

方法提供方
package com;

public class Boot{

	private String name;

	// public Boot (String name){
	// 	this.name = name;
	// }    注意这里的两个构造函数。

	public Boot (){
	}
	
	public void run(){
		System.out.println(name);
	}
}

======================
调用方
package com;
import com.Boot;
public class Cloud{

	public Cloud (){
	}

	public static void main(String[] args){
		new Boot().run();
	}
}

4、原因说明

实际使用的类文件不配套导致的异常。即通常所说的jar包版本不一致导致此问题。

5、实际举例

Spring Boot 2.x 使用 feign 出现此错误。Spring Cloud 1.1调用的构造函数是 new SpringApplicationBuilder(new Object[0]) ，但Spring Boot 2.1 提供的构造函数只有一个

public SpringApplicationBuilder(Class... sources) {
this.application = this.createSpringApplication(sources);
}

java.lang.NoSuchMethodError: org.springframework.boot.builder.SpringApplicationBuilder.<init>([Ljava/lang/Object;)V
	at org.springframework.cloud.bootstrap.BootstrapApplicationListener.bootstrapServiceContext(BootstrapApplicationListener.java:120)
	at org.springframework.cloud.bootstrap.BootstrapApplicationListener.onApplicationEvent(BootstrapApplicationListener.java:84)
	at org.springframework.cloud.bootstrap.BootstrapApplicationListener.onApplicationEvent(BootstrapApplicationListener.java:62)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.doInvokeListener(SimpleApplicationEventMulticaster.java:172)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:165)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:139)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:127)
	at org.springframework.boot.context.event.EventPublishingRunListener.environmentPrepared(EventPublishingRunListener.java:76)
	at org.springframework.boot.SpringApplicationRunListeners.environmentPrepared(SpringApplicationRunListeners.java:53)
	at org.springframework.boot.SpringApplication.prepareEnvironment(SpringApplication.java:342)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:305)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1215)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1204)
	at com.Application.main(Application.java:22)

6、解决方式：换依赖jar的版本

java.lang.NoSuchMethodError 介绍

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