多次调用java.awt.Toolkit.getDefaultToolkit方法获得是同一个单例的Toolkit

java.awt.Toolkit.getDefaultToolkit()

import java.awt.Toolkit;

public class 多次调用Toolkit的getDefaultToolkit方法获得是同一个单例的Toolkit {
	
	static public void main (String...arguments)throws Exception{
		
		System.out.println("Toolkit.getDefaultToolkit()获得的java.awt.Toolkit是不是同一个? 是否为单例设计模式?");
		System.out.println("Toolkit.getDefaultToolkit()==Toolkit.getDefaultToolkit()的结果是");
		System.out.println(Toolkit.getDefaultToolkit()==Toolkit.getDefaultToolkit());
		
		for(int r=0; r<100; r++) {
			for(int c=0; c<100; c++) {
				System.out.print(Toolkit.getDefaultToolkit()==Toolkit.getDefaultToolkit());
			}
			System.out.println();
		}
		
	}

}

jdk17源码去掉注释的

    private static Toolkit toolkit;

    public static synchronized Toolkit getDefaultToolkit() {
        if (toolkit == null) {
            toolkit = PlatformGraphicsInfo.createToolkit();
            if (GraphicsEnvironment.isHeadless() &&
                !(toolkit instanceof HeadlessToolkit)) {
                toolkit = new HeadlessToolkit(toolkit);
            }
            if (!GraphicsEnvironment.isHeadless()) {
                loadAssistiveTechnologies();
            }
        }
        return toolkit;
    }

private static Toolkit toolkit; 一个镜头变量保存实例, 如果保存实例的变量为空, 才创建新实例. 所以是单例

jdk17源码带注释的

    /**
     * The default toolkit.
     */
    private static Toolkit toolkit;

   /**
     * Gets the default toolkit.
     * <p>
     * If a system property named {@code "java.awt.headless"} is set
     * to {@code true} then the headless implementation
     * of {@code Toolkit} is used,
     * otherwise the default platform-specific implementation of
     * {@code Toolkit} is used.
     * <p>
     * If this Toolkit is not a headless implementation and if they exist, service
     * providers of {@link javax.accessibility.AccessibilityProvider} will be loaded
     * if specified by the system property
     * {@code javax.accessibility.assistive_technologies}.
     * <p>
     * An example of setting this property is to invoke Java with
     * {@code -Djavax.accessibility.assistive_technologies=MyServiceProvider}.
     * In addition to MyServiceProvider other service providers can be specified
     * using a comma separated list.  Service providers are loaded after the AWT
     * toolkit is created.
     * <p>
     * If the list of assistive technology providers as provided through system
     * property "{@systemProperty javax.accessibility.assistive_technologies}"
     * is the empty string or contains only
     * {@linkplain Character#isWhitespace(int) white space} characters it is
     * ignored. All other errors are handled via an AWTError exception.
     * <p>
     * The names specified in the assistive_technologies property are used to query
     * each service provider implementation.  If the requested name matches the
     * {@linkplain AccessibilityProvider#getName name} of the service provider, the
     * {@link AccessibilityProvider#activate} method will be invoked to activate the
     * matching service provider.
     *
     * @implSpec
     * If assistive technology service providers are not specified with a system
     * property this implementation will look in a properties file located as follows:
     * <ul>
     * <li> {@code ${user.home}/.accessibility.properties}
     * <li> {@code ${java.home}/conf/accessibility.properties}
     * </ul>
     * Only the first of these files to be located will be consulted.  The requested
     * service providers are specified by setting the {@code assistive_technologies=}
     * property.  A single provider or a comma separated list of providers can be
     * specified.
     *
     * @return     the default toolkit.
     * @throws  AWTError in case of an error loading assistive technologies.
     * @see java.util.ServiceLoader
     * @see javax.accessibility.AccessibilityProvider
     */
    public static synchronized Toolkit getDefaultToolkit() {
        if (toolkit == null) {
            toolkit = PlatformGraphicsInfo.createToolkit();
            if (GraphicsEnvironment.isHeadless() &&
                !(toolkit instanceof HeadlessToolkit)) {
                toolkit = new HeadlessToolkit(toolkit);
            }
            if (!GraphicsEnvironment.isHeadless()) {
                loadAssistiveTechnologies();
            }
        }
        return toolkit;
    }

注释翻译

获取默认工具包。

如果名为“java.awt.headless”的系统属性设置为 true，则使用 Toolkit 的无头实现，否则使用 Toolkit
的默认平台特定实现。

如果此工具包不是无头实现，并且如果它们存在，则如果由系统属性javax.accessibility.assistive_technologies指定，则将加载javax.accessibility.AccessibilityProvider的服务提供者。

设置此属性的一个示例是使用
-Djavax.accessibility.assistive_technologies=MyServiceProvider 调用 Java。除了 MyServiceProvider 之外，还可以使用逗号分隔的列表指定其他服务提供程序。创建
AWT工具包后，将加载服务提供商。

如果通过系统属性“{@systemProperty
javax.accessibility.assistive_technologies}”提供的辅助技术提供程序列表是空字符串或仅包含空格字符，则忽略它。所有其他错误都通过
AWTError 异常进行处理。

assistive_technologies属性中指定的名称用于查询每个服务提供程序实现。如果请求的名称与服务提供商的名称匹配，则将调用
AccessibilityProvider.activate 方法来激活匹配的服务提供程序。 返回：默认工具包。抛出：AWTError -
在加载辅助技术时出错。另请参阅：java.util.ServiceLoaderjavax.accessibility.AccessibilityProviderImpl
规范：如果未使用 systemproperty 指定辅助技术服务提供程序，则此实现将在位于以下属性文件中查找： •
${user.home}/.accessibility.properties} •
${java.home}/conf/accessibility.properties} 只会查阅要找到的第一个文件。通过设置
assistive_technologies=属性来指定请求的服务提供程序。可以指定单个提供程序或以逗号分隔的提供程序列表。