demo: Update slot chain SPI demo (alibaba#1581)

Signed-off-by: yunfeiyanggzq <yunfeiyang@buaa.edu.cn>

sentinel-demo/sentinel-demo-slot-chain-spi/src/main/java/com/alibaba/csp/sentinel/demo/slot/DemoSlot.java

@@ -19,17 +19,19 @@
 import com.alibaba.csp.sentinel.node.DefaultNode;
 import com.alibaba.csp.sentinel.slotchain.AbstractLinkedProcessorSlot;
 import com.alibaba.csp.sentinel.slotchain.ResourceWrapper;
+import com.alibaba.csp.sentinel.spi.SpiOrder;
 
 /**
  * An example slot that records current context and entry resource.
  *
  * @author Eric Zhao
  */
+@SpiOrder(-3500)
 public class DemoSlot extends AbstractLinkedProcessorSlot<DefaultNode> {
 
     @Override
     public void entry(Context context, ResourceWrapper resourceWrapper, DefaultNode node, int count, boolean prioritized, Object... args)
-        throws Throwable {
+            throws Throwable {
         System.out.println("Current context: " + context.getName());
         System.out.println("Current entry resource: " + context.getCurEntry().getResourceWrapper().getName());
 

sentinel-demo/sentinel-demo-slot-chain-spi/src/main/java/com/alibaba/csp/sentinel/demo/slot/DemoSlotChainBuilder.java

@@ -20,17 +20,11 @@
 import com.alibaba.csp.sentinel.slots.DefaultSlotChainBuilder;
 
 /**
- * An example slot chain builder. To activate this slot chain builder,
- * add the class name to corresponding SPI file in `resource/META-INF/services` directory.
- *
  * @author Eric Zhao
+ *
+ * @deprecated since 1.7.2, we can use @SpiOrder(-3500) to adjust the order of {@link DemoSlot},
+ * this class is reserved for compatibility with older versions.
  */
-public class DemoSlotChainBuilder implements SlotChainBuilder {
-
-    @Override
-    public ProcessorSlotChain build() {
-        ProcessorSlotChain chain = new DefaultSlotChainBuilder().build();
-        chain.addLast(new DemoSlot());
-        return chain;
-    }
+@Deprecated
+public class DemoSlotChainBuilder extends DefaultSlotChainBuilder {
 }

sentinel-demo/sentinel-demo-slot-chain-spi/src/main/resources/META-INF/services/com.alibaba.csp.sentinel.slotchain.ProcessorSlot

@@ -0,0 +1,2 @@
+# Custom slot processor
+com.alibaba.csp.sentinel.demo.slot.DemoSlot